Revox A700 Repro amp upgrade - UK Vintage Radio Repair and Restoration Discussion Forum

TIMTAPE · 5th Nov 2015, 12:05 pm

I have a lovely old A700 which works well but compared to say an A77 or B77 or many other quality reel to reels, its playback preamp noise is excessive. From reading it seems the original 1970's preamp IC, the TBA 931 was known to be noisy. It seems a shame to have such a well made machine compromised in its playback performance.

The LM833 chip is not exactly the latest design but promised to give lower noise than the original. Unfortunately the pinouts are different but the LM833 only has 8 pins compared to the original's 14 pins giving a little more room to fit the necessary jumper wiring. Pictured is my prototype.

I havent quantified the improvement but there is an obvious reduction in noise. I havent noticed any increased tendency to magnetise the repro head but just to be sure I demag it before playing an important tape. The circuit has been in use for a year now although the machine doesnt get a lot of use.

Hope you find the mod interesting especially if you own one of these nice old machines.

Tim

paulsherwin · 5th Nov 2015, 2:02 pm

Given that you've fitted a socket you could experiment with some other audio op-amps to see if you can get a further improvement. The LM833 is a decent enough part but there are higher performance alternatives which should just plug in.

TIMTAPE · 5th Nov 2015, 7:09 pm

Good point Paul, thanks.

llama · 5th Nov 2015, 7:21 pm

Please let us know your findings.

Some while ago I mentioned that a Ferrograph, when fitted with a third head, required a step-up transformer. Someone (and I can't find the postings now) stated that a suitable modern approach would be to use an op-amp instead of the transformer and follow that with an appropriate playback EQ stage.

Can anyone remember what the recommended IC was, please?
Graham

TIMTAPE · 5th Nov 2015, 7:41 pm

I think the old Series 5 Ferro used a step up transformer even for the standard record/play head which interestingly had two windings, high impedance for record and low impedance for playback, followed by the transformer feeding an EF86, which fed a passive EQ circuit.
Many years ago, and on the theme of lowering noise, I bypassed the step up transformer and EF86 and used a two transistor stage instead. It worked well. I'm sure various modern op amps would be even better than my discrete design.

5th Nov 2015, 12:05 pm	#1
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Revox A700 Repro amp upgrade I have a lovely old A700 which works well but compared to say an A77 or B77 or many other quality reel to reels, its playback preamp noise is excessive. From reading it seems the original 1970's preamp IC, the TBA 931 was known to be noisy. It seems a shame to have such a well made machine compromised in its playback performance. The LM833 chip is not exactly the latest design but promised to give lower noise than the original. Unfortunately the pinouts are different but the LM833 only has 8 pins compared to the original's 14 pins giving a little more room to fit the necessary jumper wiring. Pictured is my prototype. I havent quantified the improvement but there is an obvious reduction in noise. I havent noticed any increased tendency to magnetise the repro head but just to be sure I demag it before playing an important tape. The circuit has been in use for a year now although the machine doesnt get a lot of use. Hope you find the mod interesting especially if you own one of these nice old machines. Tim Attached Thumbnails

5th Nov 2015, 2:02 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,944	Re: Revox A700 Repro amp upgrade Given that you've fitted a socket you could experiment with some other audio op-amps to see if you can get a further improvement. The LM833 is a decent enough part but there are higher performance alternatives which should just plug in.

5th Nov 2015, 7:09 pm	#3
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox A700 Repro amp upgrade Good point Paul, thanks.

5th Nov 2015, 7:21 pm	#4
llama Octode Join Date: Jun 2006 Location: St Osyth, Nr Clacton, Essex, UK. Posts: 1,482	Re: Revox A700 Repro amp upgrade Please let us know your findings. Some while ago I mentioned that a Ferrograph, when fitted with a third head, required a step-up transformer. Someone (and I can't find the postings now) stated that a suitable modern approach would be to use an op-amp instead of the transformer and follow that with an appropriate playback EQ stage. Can anyone remember what the recommended IC was, please? Graham __________________ Half my stuff is junk - trouble is, I don't know which half!

5th Nov 2015, 7:41 pm	#5
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox A700 Repro amp upgrade I think the old Series 5 Ferro used a step up transformer even for the standard record/play head which interestingly had two windings, high impedance for record and low impedance for playback, followed by the transformer feeding an EF86, which fed a passive EQ circuit. Many years ago, and on the theme of lowering noise, I bypassed the step up transformer and EF86 and used a two transistor stage instead. It worked well. I'm sure various modern op amps would be even better than my discrete design.