Hello and welcome to whatishifi blog! This time I am hosting again a post from Panos. After finishing moding a Parasound HCA-2200II power amplifier, time has come for an easier mod at a ROTEL RC-970BX pre amplifier. This mod led to an interesting comparison test of op-amps, you will find it at the end of this post.
Description of the PCB
The top sub-circuit (behind the big capacitors) with the relay is the protection circuit.
NOTE: This mod can be done to the following models of Rotel preamplifiers:
Last but not least I replaced the 50K volume pot with a stepped pot. This pot is directly connected between the input selector and the tone on / off switch. A small tip: when putting the new one in, screw the pot on the support face plate first and then solder it.
You can search eBay for: “DACT Type SMD Stepped Attenuator 21 step volume control pot D Shape Shaft".
So, from now on it is Panos' words!
Chris
Chris
Moding (upgrading) a humble ROTEL RC-970BX 2ch stereo pre amplifier.
Let me start by saying that I have a bad habit like an addiction: I spend a lot of time surfing on eBay looking for various
things that interest me (eg. HiFi components, photography equipment, electronics etc.).
When I spot something within my interests that has a ridiculous low price, I place a
bid just for fun and in order to see the price that the item will sell. More than once I
have unexpectedly won items I don't really need at a very low price.
One of these items was a Rotel RC-970BX. It was in
a very good cosmetic condition, but there was a problem with the volume pot; when
rotated, at 11 o’clock position it had a "scratchy" feeling but without any audible or other kind of side effects. Nevertheless, I decided to open it and spray some contact lubrication spray. With the top cover of the amp removed, I noticed that this amp has a simple but efficient circuit built around three op-amps, so it looked like an easy project for mods and upgrades.
 |
| Inside view of the Rotel RC-970BX pre amp. |
The top sub-circuit (behind the big capacitors) with the relay is the protection circuit.
Underneath is the dual / symmetrical class A shunt regulator power supply.
Under the power supply there are three dual OpAmps. From left to right you have:
- IC601 - NJM4556AD for the headphone output.
- IC502 - NJM2114 for the tone control.
- IC501 - NJM4558 for the volume control.
According to the schematic, all the op-amps are in typical non inverting configuration. Looking at the components on the PCB, I realized there was no need to change the power supply electrolytic capacitors since they are of good quality (Rubicon), but I could change the OpAmps with something better. So the following mods / upgrades were made:
- I replaced the two electrolytic caps (C509 - C510) from the top and bottom of the IC501 with two 1000μF/25V for better power filtering.
- I removed the four coupling electrolytic capacitors (C501 - C502 - C505 - C506) and replaced them with jumper wires; they are in the signal path and they are not needed since this is a symmetrical design.
- I installed an 8 pin DIP base to IC501 for easy OpAmp rolling and testing.
 |
| This DIP base allows intant changing of op-amps. |
RC-970BX, RC971, RC980BX (positions IC501, C501, C502, C505, C506).
RC-972 (positions IC503, C523, C524, C527, C528).
RC-990BX (only the four capacitors since it is a discrete design, positions C501, C502, C507, C508).
Update: As suggested from Fabio in the comments section, we forgot to include RC-960BX. Thank you Fabio!
Then I turned my attention to the other OpAmp (IC502). I replaced the four electrolytic coupling capacitors that are in the signal path with Nichicon muse bipolar and I installed an 8 pin DIP base - maybe this was not necessary since this part of the circuit is just for the tone controls but since the amp was on the bench I couldn't resist.
 |
| Tone control mods - not a must since I don't use tone controls, but I could not resist. |
 |
| The new step volume control. |
All mods done.
 |
| Everything done! |
Now is time for OpAmp rolling to test if there
is any difference between OpAmps, many people are sworn enemies of OpAmps (OK, a discreet design can virtually have sky high performance), but
the truth is that there are hundreds of these in a studio mastering console.
Panos
Christos writing: We were very excited to proceed with the test regarding the sound performance of different op-amps. The 8 pin DIP base that Panos installed made the test very easy; we could change op-amps in a few seconds and compare the sound with no fuss at all.
Before giving you the details about the op-amp test, let me mention that with the stepped volume control that Panos installed we measured at the oscilloscope a perfectly balanced output from both channels - even an Alps Blue potentiometer made in Japan has a small imbalance at the output level of the two channels.
We choose to listen to a vinyl album (Stan Getz / João Gilberto Featuring Antonio Carlos Jobim – Getz / Gilberto, Verve Reissues / Quality Record Pressings / Analogue Productions B0015625-01), it is the very same album that we used at the battle of analogue Vs. digital audio we made at this post. As usual, we stuck to our reference custom made phono stage - the Rotel has only line inputs. Power amp was a Parasound Halo A21.
We started with the original (NJM4558) op-amp that came with the Rotel. And we stopped right away! Compared to our reference pre amp, the sound was awful. As soon as we heard the first vocals, we felt like there was a lot of hubbub giving us a headache. So this op-amp is definitely a big and fast no.
Second at our list was the highly appreciated Burr-Brown OPA627AP. Things dramatically improved, the headache disappeared. Yes, nothing was disturbing, we can live with this sound. Rotel owners, this is a must do upgrade.
Third in the list was the Burr-Brown OPA627AU. Things got a little lazy and the low end lacked the energy that the OPA627AP was delivering but still nothing was disturbing. This op-amp is a yes but still, if you do the upgrade there is no reason why not to go for the OPA627AP.
Next on the list was the ST LF351N. It delivered a forward soundstage, sssssss kind of sound and was edgy on the high frequencies. It is a no.
Last but not least was the AD 817AN. Even though it was an improvement over the original op-amp, it is a no. Just go for the OPA627AP.
Now, a world of caution: It looks like that the market is flooded with fake OPA627. Our Chinese friends thought that it is a good idea to print OPA627AP budge at a similar op-amp. If you decide to get one, get it from an official BB distributor.
How much did the Rotel benefit from the OPA627AP? Really a lot. It was transformed from an unpleasing sounding device to a good pre amp. There was nothing to complain about, we enjoyed listening to it. The only thing that was lacking was the 3D soundstage that our usual preamp offers - the sound it produces literally hugs you and the Rotel could not provide that. Still, listening to the Rotel with the OPA627AP was really pleasant and we are comparing apples with oranges here; even though a DIY device, our reference pre-amp is a rather exotic component regarding it's performance.
Our next project is testing if the OPA627AP will do any good at a Sony Mini Disc player. As we found out, changing the op-amp does not always means better sound: at the Rotel it made a difference, at the Sony it didn't.
Update: One question we got is why we didn't get any measurements. Well, we could do, but on the other hand the pdf data sheets of the op-amps have all the specifications - I guess confirming them is not of much interest. Also, better specs do not necessarily mean better sound. I mean, if one amp has 0.006% THD and another amp has 0.008 THD, it is not of much interest. Of course, if one amp has 2% THD then probably there is something wrong. So, we just wanted to understand if the op-amp would make any difference in sound and which op-amp sounds better, without being affected by specs and that is what we did. We trust our ears, we try to be down to earth & objective and we ask you to trust us, but hey! If you don't, and provided you have the skills and knowledge for some mods, I can only encourage you to make the test yourself - we would love to hear your comments.
Happy listening as always!
Chris
P.S. If you liked this post, you will probably like most of this blog. Why not having a look at all the things we have wrote about here.
We will test the following OpAmps: The original NJM4558, the BB OPA2134PA, and the
highly appreciated OPA627.
Panos
Christos writing: We were very excited to proceed with the test regarding the sound performance of different op-amps. The 8 pin DIP base that Panos installed made the test very easy; we could change op-amps in a few seconds and compare the sound with no fuss at all.
Before giving you the details about the op-amp test, let me mention that with the stepped volume control that Panos installed we measured at the oscilloscope a perfectly balanced output from both channels - even an Alps Blue potentiometer made in Japan has a small imbalance at the output level of the two channels.
 |
| Our op-amps waiting to be compared. |
We started with the original (NJM4558) op-amp that came with the Rotel. And we stopped right away! Compared to our reference pre amp, the sound was awful. As soon as we heard the first vocals, we felt like there was a lot of hubbub giving us a headache. So this op-amp is definitely a big and fast no.
Second at our list was the highly appreciated Burr-Brown OPA627AP. Things dramatically improved, the headache disappeared. Yes, nothing was disturbing, we can live with this sound. Rotel owners, this is a must do upgrade.
Third in the list was the Burr-Brown OPA627AU. Things got a little lazy and the low end lacked the energy that the OPA627AP was delivering but still nothing was disturbing. This op-amp is a yes but still, if you do the upgrade there is no reason why not to go for the OPA627AP.
Next on the list was the ST LF351N. It delivered a forward soundstage, sssssss kind of sound and was edgy on the high frequencies. It is a no.
Last but not least was the AD 817AN. Even though it was an improvement over the original op-amp, it is a no. Just go for the OPA627AP.
Now, a world of caution: It looks like that the market is flooded with fake OPA627. Our Chinese friends thought that it is a good idea to print OPA627AP budge at a similar op-amp. If you decide to get one, get it from an official BB distributor.
How much did the Rotel benefit from the OPA627AP? Really a lot. It was transformed from an unpleasing sounding device to a good pre amp. There was nothing to complain about, we enjoyed listening to it. The only thing that was lacking was the 3D soundstage that our usual preamp offers - the sound it produces literally hugs you and the Rotel could not provide that. Still, listening to the Rotel with the OPA627AP was really pleasant and we are comparing apples with oranges here; even though a DIY device, our reference pre-amp is a rather exotic component regarding it's performance.
Our next project is testing if the OPA627AP will do any good at a Sony Mini Disc player. As we found out, changing the op-amp does not always means better sound: at the Rotel it made a difference, at the Sony it didn't.
Update: One question we got is why we didn't get any measurements. Well, we could do, but on the other hand the pdf data sheets of the op-amps have all the specifications - I guess confirming them is not of much interest. Also, better specs do not necessarily mean better sound. I mean, if one amp has 0.006% THD and another amp has 0.008 THD, it is not of much interest. Of course, if one amp has 2% THD then probably there is something wrong. So, we just wanted to understand if the op-amp would make any difference in sound and which op-amp sounds better, without being affected by specs and that is what we did. We trust our ears, we try to be down to earth & objective and we ask you to trust us, but hey! If you don't, and provided you have the skills and knowledge for some mods, I can only encourage you to make the test yourself - we would love to hear your comments.
Happy listening as always!
Chris
P.S. If you liked this post, you will probably like most of this blog. Why not having a look at all the things we have wrote about here.
Hello Christos thanks for the great review!!.
ReplyDeleteAfter reading this iam going also upgrade my RC-971 with the Burr-Brown. Can I replace all three opamps with the same version?
Thanks for your reply!!
Hello Michel, thank you for your kind words. You can also replace the opamp for the tone controls with the same version; however if you don't use tone controls there is no need to do so (if you put the "tone" switch at the off position, this opamp is not involved in the signal path). Now the third opamp is for driving the headphones, here you can not use the same opamp because it has not enough gain. Unfortunately we can not make any suggestions, we haven't done a test for headphones.
DeleteChristos
Hello Christos thanks for your reply! The Burr-Browns are ordered, 99pct the tone is disabled but i replaced them both. I found also some discrete designs iam curious how these sounds. Keep you posted!
DeleteHello Michael, looking forward for your test results.
DeleteHi
DeleteI tried this but did not realise that the Burr Brown is a single channel so will have to purchase another along with the dual channel adapter. My RC970bx has the OP275 op amp on the volume control as standard. Will 2 OPA627 x 2 still be compatiable with an adapter board?
Also do you think I may have damaged my single OPA627 by placing it directly into a dual channel socket?
Hi,
DeleteYes you must have two 627 with an adaptor, (see the last photo in the post), and yes all the opamps are pin to pin compatible, also no damage should occur since the power supply V- is the same on all opamps (single and dual) and the V+ is pin 8 on dual and not used on single.
A good practice is to read the datasheet of components.
Regards
Panagiotis
hi, what is the value of the DACT Type SMD Stepped Attenuator 21 step volume control pot D Shape Shaft?
DeleteHello, same value as the original; 50k.
Deleteplz help me i burned the power suply i want the voltage plz plz
ReplyDeleteHello, that is strange, the fuses should protect your power supply. At the transformer output of my RC-970BX I measure 22-0-22V AC approx. The op amps get +/-18V DC through the zeners. Hope it helps!
ReplyDeletethank you alot my friend now i found an other broken hf relay the blue obe in the pictur above rk1-12v could i change it whit rel30012 if you ask about how i burned it i change the v in wires from 220 to 120 by mistake and thank you so much for the help
ReplyDeletethe protection circuit whit the relay is broken i got 14v from the preamp output it burned my left speaker can i know if i can bypasse or fix the protection circuit plz
ReplyDeleteIf you know electronics you can download the manual from here:
ReplyDeletehttps://www.hifiengine.com/manual_library/rotel/rc-970.shtml
A free registration is all you need, If you do not know how to follow the schematics and fix the problem, then you should take it to a professional to fix it for you.
It is not wise to bypass the protection circuit.
Regards
Hi Guys.
ReplyDeleteYou haven't listed the RC-960BX, which I have, in the list of modifiable preamplifiers. Why? It has just the Phono input more than the others you mentioned.
Thanks.
Fabio
Hello Fabio, you are right. RC-960BX looks the same so it can be moded the same way. Thank you for bringing this up.
DeleteHello Christos and Panos.
DeleteYour blog is very nice.
Finally I had time to study my Rotel RC-960 BX schematics. Please, can Panos tell me why the negative feedback R209 (960) is 10k, and R511 (970) is 2k74, since the op amps are in the same configuration? Thanks.
Fabio from Rome, Italy.
Actually the RC-960 and the 970 schematic reports the NE5532AN rather then NJM4558 as you stated. Are they equivalent?
DeleteHi Fabio,
DeleteRegarding your first question the mod is not affected by the negative feedback resistor, I do not know why there is a Deference.
For the second question, yes they are equivalent, probably Rotel used what they had at the time.
Regards
Thank you, Panos.
DeleteRegards
Hi Guys,
DeleteI waited more than one month to receive the dual to single adapter from Hong Kong, but finally I got it.
Quickly inserted the two OPA Burr Brown and... Wow! It seems that before there was a curtain in front of the speakers. The mid-high range is more transparent, and the low end is impressively granitic, more deep and stable!
I was afraid being placebo-effect victim, so I switched on the tone control, just to add the NE5532AN to the signal path, and the curtain is back again. No placebo at all!
I've just read the issue by Frank Jamelle: You're right Frank, gettin out the capacitor from the signal path could be dangerous if DC occours at the input. But I'm confident that decoupling caps are at the power amps inputs (HK Citation 22/HK 770).
About the supply voltage: in my RC960BX I got +/- 17.4V. After hours of listening I've put my finger on the OPA and they are pretty cold, even colder than the original NE55342AN. We'll see on the long run... anyway, thanks a lot for the alternative options!
This comment has been removed by the author.
DeleteHello Fabio, nice to know that you like the mod. Thank you for sharing your impressions.
DeleteWhat about lampizating the pre?
ReplyDeleteHi,
Deletelampizating the pre is doable but it is more complicated, you need to add an extra high voltage transformer with multiple secondaries - bridge rectifier etc. This mod is simple for someone with basic soldering skills.
If someone have the skill to preform a lampizating mod he can build a pre from scratch.
Regards
Thank you for your comment. Generally, we like tube sound but we believe that the best place for a tube is at the pre amp and not the power amp. In particular, we haven’t tried it at the Rotel so we have no opinion.
ReplyDeleteHi,
ReplyDeleteI have modified 2 Rotel RB-970BX.When you have opened the Chassis delete all Electrolytics of the power supply.The Quality of them was good, but they are 20 years old and may have lost capacity. I measured one of the coupling electrolytics ( black gate ) and got 0,9uF instead of 10uF.At digikey you can select electrolytics - chose those with long lifetime at e
levated temperatures and low ESR ( impedace ).
Deleting of the coupling capacitors may work, but depending on your power-amplifier any dc from sources may kill your speakers.I used WIMA MKT of 6,8uF because of 5mm spacing, but you can use also use PP,PS and others in the range of 5-20uf ( the input impedance of the rotel is quite high ).
The power supply of the Rotel is +-18V, but I have measured 18,5 one time on positive supply. This is beyond the absolute maximum rating of the OPA627 - and even +-18V may not bee good for longtime reliability of the OPA627. If you need fet-input opas use OPA1642 or 2 OPA827. I have used OPA1612 and wow!!!!.Show me any discrete design with better specs.!!!
Frank
Hello Frank, thank you for your comments.
ReplyDeleteHi Frank.the opa 1612 did a big improvement in sound?it was rotel RC 970 cus the RB 970 Don t have op amps
ReplyDeleteHi i m jacky,recently bought a rotel RB 970BX, it a 110 voltage version may i know is it can convert it to 230 voltage for my local use? I checked the service manual schematic diagram, just want to confirm it with the helps with those experts here,, thank so much for helps, urgently for your confirmation here thanks.,can email me at jackytah61@gmail.com also
ReplyDeleteHi,
DeleteFirst of all, I do not have this device so please be extra careful - Ican't take any responsibility of possible damage!
Looking at the schematic diagram, you have to remove two jumper wires from the small board named TB03114 marked 115v (AC3-AC4 & AC5-AC6) and place a jumper wire where the 230V mark is AC4-AC5).
What puzzles me is that in the schematic diagram it does not show where the 220V primary is wired on the board.
I strongly recommend that after the conversion to remove the two fuses (F902-F901) and measure with a multi-meter in VAC setting, between the black and red wires going on the board; judging from the smoothing capacitors that are rated at 50V (please check yours) you must get 40-45V AC. This way you will be able to understand if everything worked fine, without causing any damage to the amp.
Hello and thanks for your work and valuable information :-) I have in mind to mod my RC 970 BX, with the BB OPA627AP. I found them on the web. But I am not sure about the "dual channel adapter". Would you have a link to suggest? (I am in Europe, Belgium) Regarding the other component: is a "8 pin DIP base" needed?
ReplyDeleteThank you for your support.
All the best,
Thierry
Hi,
DeleteI ordered my from eBay here is an example.
https://www.ebay.co.uk/itm/Gold-plated-DIP8-Mono-To-Dual-OP-Amp-Adapter-Converter-Socket-AD797-AD847-OPA128/263334358088?hash=item3d4ff36c48:g:nfEAAOSwcA1cyhpJ
Regards
A base is not needed you can solder the adapter directly on the PCB but is a convenient way in case you want to change the op amp.
DeleteHello Panos,
DeleteMany thanks for your replies! It will help, definitely...
Have a nice day !
Thierry
Hello and thank you for this wonderful information. I have a question about the modification to the volume pot change to stepped attenuator. Originally the rc-970/971 had dual shaft volume control which served as a Balance control. Did your modification result in no ability to control R/L channel volume? Is there an upgrade that would allow for the separate channel volume control feature to be maintained?
ReplyDeleteThank you for your thoughts.
-jeff
Hello Jeff, thank you for your comment. You are right, with our modification at the volume control the balance control is lost. We are not aware of a stepped volume pot with dual shaft, but to be honest we haven't searched really hard for one, because we don't use the balance control. We can't think of a pot upgrade that would allow to keep balance control and be practical to implement it.
DeleteHello! I've just been reading the info above and am keen to carry out some work on my RC971 as it's still at factory spec.
ReplyDeleteA seatch on ebay for the volume pot you mention only shows ones in China. Is this probably the only way to get hold of this specific model? I haven't checked with electronic component retailers yet but am not sure if you could a suitable alternative? Many thanks, Alex
Hello Alex, unfortunately we are not aware of another source - we got ours from China also.
DeleteHi
ReplyDeleteI have done various parts of this mod over time. I did the op amp about a year ago and did the volume control today (spare time with the virus lock down). A few questions. Loudness when increasing the volume seems lower and get louder later. Is this because the original volume control is front loaded to sound louder earlier? Also as the shaft of the new pot is shorter the volume knob is sunk in further - not a problem but just checking I have done it correctly.
Thanks
For the first question I have not noticed such behavior with loudness, for the second yes the knob is sunk in compared to the original pot.
ReplyDeleteHi
DeleteI think ssi means he needs to turn the knob further to get to his usual listening volume than he had to prior to the mod... I am planning to carry out the op amp and volume pot mods but don't understand the capacitor mods... You can just remove the 220uF caps and replace with 1000uF? And get rid of the coupling caps completely? I can manage to swap one part for another but I don't fully understand circuits so not sure about this... I was going to replace the coupling caps with polyprop caps... plenty of room for them on the board...
The small capacitors are for DC blocking, since the power supply is symmetrical no DC offset is present so they are not necessary, also replacing them with a good quality caps would make a difference but caps always have there own sound, I ran the pre with this mods and I haven't had a problem.
DeleteHi, I have a RC 970 BX preamplifier, funny things is, the tone on/off switch having a cracking sound frim speaker when u try to turn on or off, then the whole Amp is dead, not signal passing out at all, but if you leave it there for a day or two, it works back again, please advice, tq
ReplyDeleteHi, difficult to guess what is wrong. When you engage the tone controls, signal passes through additional op amp that takes care of tone control. Disconnect your speakers and listen if when you turn the tone switch to on, the protection relay inside your pre amp makes a click sound, meaning that it sees a fault and shuts off the signal output. Still, I don't understand why it takes a day for the pre amp to work again - maybe a capacitor is not discharging as fast it should and keeps the protection on longer than it should.
DeleteTq sir, it does make a click sound, anyway to solve the problems or just change the op amp?
DeleteHelp me understand. The amp shuts off when you move the tone switch from off to on? Or from on to off? Or both?
DeleteI had the same problem with the tone selector switch rc 970bx pre amp.
DeleteIs there anyway I can buy the tone control selector switch?
Don't really know. I would check part number from the service manual or take it out and go to a store and see what I can find.
DeleteChris, did you ever listen to the 627AP WITH the 4 DC blocking caps in place? They shouldn't have more than millivolts across them, as such the can't produce distortion or anything else in the signal. A "veiled" sound should be coming from the original opamps and perhaps those caps if they're bad/too old, also an aging power supply. Originals are nearly 30 years old so I'm going to shotgun all the 'lytics. Too bad, my opamps coupling/blocking caps are Rubicon Black Gates...
ReplyDeleteI'll a/b mine when I do the opamps swap, w/good Nichicons and without. I'm trying the LME49860 in both the tone control and tone defeat circuits, a hi voltage LME49720, it can run at 22v and has a 44v max.
Also, importantly, forget 0.1-.01uf cermaic decoupling caps at the V+ and V- pins on the DIP8 socket to ground, these newer fast opamps will oscillate without them. Get the cap body close to the socket leg and make the ground lead as short as possible.
All I've done to my RC971 so far is get the covers off, I'll measure the working voltage and any swing on the original chips. I've got a OP275GP in the tone defeat circuit which has a max input voltage of 22v as does the JRC2114D in the tone control circuit, it's why I settled on the 49860, or I'd use the 49720 or 49723 (way cheaper.) OPA1612 is specfied 18v and absolute max at 40v, it's an incredible bipolar. OPA627AP specified is 15v and ab. max supply is 18.5v, it would sit well in the headphone circuit. Or downregulate the PSU??
I wish this OP275GP was better than I think it is, it has a lot of fans. Starts bipolar, goes FET when you turn up the gain, how cool is that? Maybe it'll wake up after I do the caps and hunt down all the drifted resistors.
Hello, thnx for stopping by. I am afraid we didn't listen with the DC blocking caps in place. If / when you do your op-amp comparison, you are mostly welcome to share your experience.
DeleteΚαλησπέρα είμαι ο John καλή χρονιά σε όλους. Έχω τον 971 και θέλω να παραγγείλω τα OPA627AP.Για να καταλάβω αυτά είναι 1ch και πρέπει να παραγγείλω 2 κομμάτια? μαζί και αντάπτορα από ένα σε δύο?
DeleteΕίπατε να βάλουμε BB αν είναι Texas instruments είναι το ιδιο
σε ποιότητα?
Καλησπέρα, καλή χρονιά! Ναι, σωστά, 2 OPA και μια βάση. H BB ανήκει στην Texas Instruments. To συγκεκριμένο ΒΒ OPA627AP μας έδωσε τον καλύτερο ήχο από αυτά που δοκιμάσαμε.
DeleteΣας Ευχαριστώ!
ReplyDeleteI m using a burson vivid 6 opamp in my rc 970 mkII and the soaund is excelent. I replaced the coupling capacitors with film capacitor from Jantzen. Execlent value
ReplyDelete