I have been restoring an old integrated amplifier.
I found various problems which have now been remedied, and I have replaced all the electrolytic capacitors, transistors, and both ICs.
I think that I have finally got the bias set correctly, although it seems to be a very delicate balance between distortion and running very hot indeed.
It sounds very good with CD as source through either Tape or Radio input, but output is very low when used with the MM input. I have tried a few different cartridges with no noticeable difference, and I know that CD output is always much higher than MM (and probably higher than tape or tuner outputs when the amp was made) Since it is not very powerful, but still sounds very good with a high level CD signal, my question is: can anything be done to modify the MM input circuit to increase the gain? I am not sufficiently knowledgeable to be able to understand the circuit, but I am able to use a soldering iron and DMM, and I would rather modify (if possible) the circuit than use a separate phono stage, and use it principally for playing records not CDs.
I would be very grateful for any advice!
I found various problems which have now been remedied, and I have replaced all the electrolytic capacitors, transistors, and both ICs.
I think that I have finally got the bias set correctly, although it seems to be a very delicate balance between distortion and running very hot indeed.
It sounds very good with CD as source through either Tape or Radio input, but output is very low when used with the MM input. I have tried a few different cartridges with no noticeable difference, and I know that CD output is always much higher than MM (and probably higher than tape or tuner outputs when the amp was made) Since it is not very powerful, but still sounds very good with a high level CD signal, my question is: can anything be done to modify the MM input circuit to increase the gain? I am not sufficiently knowledgeable to be able to understand the circuit, but I am able to use a soldering iron and DMM, and I would rather modify (if possible) the circuit than use a separate phono stage, and use it principally for playing records not CDs.
I would be very grateful for any advice!
Decrease R20, if you're sure you won't connect any high level source to it ever again.
Best regards!
Best regards!
If you close the switch connected to R23 it should give you the required gain for MM inputs.
Very nice simple looking circuit - what amp is it?
Very nice simple looking circuit - what amp is it?
Thanks for the replies!
Kay - what would you recommend for R20? And would that mean not connecting CD to either Tape or Radio any more?
Bonsai - R23 is connected to the Ceramic cartridge input and is disconnected as soon as Magnetic is selected. I will reveal the identity of the amp later - there is always a certain amount of prejudice against this particular manufacturer (I am sure you have none!) so I am interested to hear informed opinions on the design as a kind of blind test!
Mooly - I'm definitely using the MM input, and I used switch cleaner on all the switches. They don't come apart, but they all seem to work quietly. I will check all connections again.
Kay - what would you recommend for R20? And would that mean not connecting CD to either Tape or Radio any more?
Bonsai - R23 is connected to the Ceramic cartridge input and is disconnected as soon as Magnetic is selected. I will reveal the identity of the amp later - there is always a certain amount of prejudice against this particular manufacturer (I am sure you have none!) so I am interested to hear informed opinions on the design as a kind of blind test!
Mooly - I'm definitely using the MM input, and I used switch cleaner on all the switches. They don't come apart, but they all seem to work quietly. I will check all connections again.
I'm not sure which of both switches drawn beneath IC1A engages a RIAA network, as such networks usually consist of two resistors in series, paralleled with a capacitor each. Anyway, make sure one of these switches is thrown and working correctly if the phono input is used. If still too quiet, decrease R20 to, say, 680 Ω. Try other values, if necessary.
Best regards!
Best regards!
One question... when you say it is a lot quieter with the MM input, how quiet is it?
Could you drive the amp all the way to clipping (its maximum output before it distorts) with the volume turned up high or is it 'faulty' quiet and very low even on full volume. It wlould be odd for both channels to be the same if there were a problem.
Maybe one answer if it does actually go loud enough is to attenuate the line inputs to match them to the MM input.
Could you drive the amp all the way to clipping (its maximum output before it distorts) with the volume turned up high or is it 'faulty' quiet and very low even on full volume. It wlould be odd for both channels to be the same if there were a problem.
Maybe one answer if it does actually go loud enough is to attenuate the line inputs to match them to the MM input.
It would be surprising if this weren't the case. This amp expects 200mV or thereabouts at the line inputs for full power, not the 2v a CDP provides. This is the reason you notice such a difference with the phono input.
After CDs became commonplace there was a brief period of panic that old amps might have their input stages overloaded. Retrofitting attenuators to line inputs was common.
I have no predujices on this at all, I assure you. Given the unnecessary complexity of some of the stuff I've seen, I really am drawn to the simplicity of this amp. Components were expensive in the day, so legacy amp designers had to think carefully about how to get things done on a budget.
Re R23 - are you sure about that? When the switch is in the down position and R23(?) is selected that looks like RIAA EQ. When its in the up position, R20 is shunted by R27 and R25 - difficult to read the values.
The network with C19 must be the RIAA network, as it's the only one that defines two poles and a zero together with R21.
Yes -you are right. Here is a quick and dirty sim - I might have some of the values wrong though. The 1kHz gain is 45 dB
Here - mid-band gain essentially unchanged but the <50Hz response cleaned up. If the op can get the MM fixed, it should be ok gain wise with a 5mV cart.
With R20 = 1200 ohm, the gain must be about 7 times less than with 168 ohm, so around 27 dB rather than 44 dB at 1 kHz. That's rather low.
If you want to reduce R20 and keep the shape of the subsonic response the same, increase C15 by the same factor so that their product stays constant.
If you want to reduce R20 and keep the shape of the subsonic response the same, increase C15 by the same factor so that their product stays constant.
R20 in the original circuit looks like 1200 ohm to me, no idea why they chose such a small ratio of the feedback impedance to R20. Lower line levels and more sensitive MM cartridges? Too low open-loop gain of the op-amp?
The switch connects to R23 at the same time as the switch to the left of it on the diagram - the ceramic cartridge input. When the magnetic cartridge input switch is selected it automatically disconnects the ceramic input and connects the switch to the right of it as well, connecting R20 to C19 (is this the RIAA EQ for the magnetic input?)
R25 and R27 are 100R and 68k respectively.
As for attenuation, that may be necessary for CD - level really is too high at the moment, and if bias is right for CD it is too low for MM. I would say that maximum volume on MM is not extremely low, but not as loud as I would expect. I have not been testing it with very efficient speakers - old TDL RTL2s which happened to be handy - but even halfway to max is loud and not distorted with CD. This project is actually a nostalgia trip, as it was my first amplifier and, as I have never really had a 'eureka' moment with any particular piece of equipment, I am interested to see if that is because I have always had adequate stuff, or that I just listen to the music not the kit. Maybe a bit of both. Either way, I used it with a G800 MM cartridge (which is not high output) and EMI 13 x 8 speakers or Wharfedale Super 8s (neither of which I think are high efficiency) and it used to be loud enough to be told to turn it down!
Using it with CD shows me that it has the potential to be loud enough (I'm not expecting trouser-flapping levels...) so I'd be happy if that could be released for records. I'll try hooking it up to some Eminence 12" I have in a box somewhere to see what it's like with higher efficiency speakers.
(It's an Amstrad Integra 4000 - I saved my paper round money for a long time to buy it)
R25 and R27 are 100R and 68k respectively.
As for attenuation, that may be necessary for CD - level really is too high at the moment, and if bias is right for CD it is too low for MM. I would say that maximum volume on MM is not extremely low, but not as loud as I would expect. I have not been testing it with very efficient speakers - old TDL RTL2s which happened to be handy - but even halfway to max is loud and not distorted with CD. This project is actually a nostalgia trip, as it was my first amplifier and, as I have never really had a 'eureka' moment with any particular piece of equipment, I am interested to see if that is because I have always had adequate stuff, or that I just listen to the music not the kit. Maybe a bit of both. Either way, I used it with a G800 MM cartridge (which is not high output) and EMI 13 x 8 speakers or Wharfedale Super 8s (neither of which I think are high efficiency) and it used to be loud enough to be told to turn it down!
Using it with CD shows me that it has the potential to be loud enough (I'm not expecting trouser-flapping levels...) so I'd be happy if that could be released for records. I'll try hooking it up to some Eminence 12" I have in a box somewhere to see what it's like with higher efficiency speakers.
(It's an Amstrad Integra 4000 - I saved my paper round money for a long time to buy it)
For CD, I’d put an attenuator of 6:1 on the input you are going to use for that purpose - try 10k and 2.2k.
Can you confirm how the switches are paired with the ceramic input?
You should be able to fix the MM. The sim I posted above says the gain should be plenty. Of not, getting more gain will be easy enough.
Amstrad - I’d never have guessed. For a legacy amp, the design looks pretty neat and clean to me. I seem to recall reading hi-fi mags from the early 1970’s with all sorts of long defunct companies advertising their amps - Amstrad being one of them. 🙂
Can you confirm how the switches are paired with the ceramic input?
You should be able to fix the MM. The sim I posted above says the gain should be plenty. Of not, getting more gain will be easy enough.
Amstrad - I’d never have guessed. For a legacy amp, the design looks pretty neat and clean to me. I seem to recall reading hi-fi mags from the early 1970’s with all sorts of long defunct companies advertising their amps - Amstrad being one of them. 🙂
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Ah, that means both switches drawn beneath the gnd line are actuated the same time, as well as the pair above this line? The circuit drawing lacks some clarity here 🙄. Hence, there's some equalization for ceramic PU's also?
But what does R27 (68k) do exactly? It appears to be superfluous.
Best regards!
But what does R27 (68k) do exactly? It appears to be superfluous.
Best regards!
Sorry, when I made my last post I hadn't seen the previous five or six for some reason - I think the page didn't refresh properly. I'll get some photos of the switch arrangements and try for a better scan of the circuit diagram as soon as I can (care duties permitting...)
Thank you!
Thank you!