Hi all,
My Rotel RA-611 produces excess hiss - enough that it's audible from across the room with the volume at zero. I believe the issue is in the Tone/Preamp section, since if I pull the connectors to the main amp the hiss is gone.
I've traced through the circuit with an audio probe and it's nice and quiet until the collectors of Q204 and Q208. From this point forward there is the loud hiss in the circuit. I thought it might be the 50 year old transistors, so replaced the 2SC644s with KSC1845Fs, which made no difference. All of the voltages match the schematic, the capacitors have been replaced and the carbon resistors have been replaced with metal film - also made no difference.
Please find the schematic attached.
Many thanks in advance for the help!
My Rotel RA-611 produces excess hiss - enough that it's audible from across the room with the volume at zero. I believe the issue is in the Tone/Preamp section, since if I pull the connectors to the main amp the hiss is gone.
I've traced through the circuit with an audio probe and it's nice and quiet until the collectors of Q204 and Q208. From this point forward there is the loud hiss in the circuit. I thought it might be the 50 year old transistors, so replaced the 2SC644s with KSC1845Fs, which made no difference. All of the voltages match the schematic, the capacitors have been replaced and the carbon resistors have been replaced with metal film - also made no difference.
Please find the schematic attached.
Many thanks in advance for the help!
Attachments
If the hiss is present in both channels then you could have a noisy power supply since it is common to both channels. Maybe a filter cap in the power supply has opened. The point that has the most noise is the output of a 2 transistor amplifier which has a gain of 10 so it would have the most noise. If the amplifier seems to work fine otherwise then I would look for a noisy power supply first. You could put a 100uf capacitor or so on your B-2 power point to signal ground to see if there is a sensitivity there.
I've just tried a 100uf capacitor between B-2 and signal ground, but it didn't make any difference to the hiss. I did also try running the tone/preamp from battery power, with B-2 disconnected and still got the hiss. When I audio probe B-2 I can't hear any noise.
The amp does work fine otherwise. I don't get the noise when I play audio through the main amp in using an external device.
The amp does work fine otherwise. I don't get the noise when I play audio through the main amp in using an external device.
Are your base and treble controls working normally? Do you still get noise if you short your audio inputs to the audio ground? Push or slide all of your controls. See if there is any change in the noise. Maybe a pot or a switch has lost its ground if it has one.
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All of the tone controls work as expected when playing music. The grounding of the controls looks good - all less than one ohm. I tried shorting the inputs to ground but didn't have any effect.
These controls don't affect the noise:
Balance
Volume
Mono
Input selector
Monitor selector
These controls do affect the noise:
Treble
Tone defeat removes any effect the treble control has on the noise
High filter
Mute - gets rid of it completely
Setting speakers to off - gets rid of it
Hard to tell if the bass, loudness or low filter have any effect.
These controls don't affect the noise:
Balance
Volume
Mono
Input selector
Monitor selector
These controls do affect the noise:
Treble
Tone defeat removes any effect the treble control has on the noise
High filter
Mute - gets rid of it completely
Setting speakers to off - gets rid of it
Hard to tell if the bass, loudness or low filter have any effect.
The problem with this old design is the structure/configuration of the gain stages. The preamp section you show which includes the tone control network and high gain line stage is after the volume control. We would not do this today and would place the volume control directly in front of the power amp input and after the preamp stages.
All this means that you get the full noise of the preamp at all times and the volume control has no means of reducing this. The Mute switch attenuates the preamp output by 20db which is why it seems to eliminate the noise. That location is exactly where the volume control 'should' be placed for best noise performance.
The effect of all this is also made worse if you have modern and efficient speakers.
All this means that you get the full noise of the preamp at all times and the volume control has no means of reducing this. The Mute switch attenuates the preamp output by 20db which is why it seems to eliminate the noise. That location is exactly where the volume control 'should' be placed for best noise performance.
The effect of all this is also made worse if you have modern and efficient speakers.
Curiously the specifications also give a not usually specified 'residual noise' value. It may not sound much at 1.5mv but is is.
Many thanks for the info! My speakers are Q Acoustic 3010s.
Not sure if it's a good idea, but would a possible solution be to reduce the gain of the high gain line stage?
Or to add a few -db of permanent muting after that stage somehow?
Not sure if it's a good idea, but would a possible solution be to reduce the gain of the high gain line stage?
Or to add a few -db of permanent muting after that stage somehow?
Adding a little attenuation is probably the easiest. I'm guessing you never use the mute function and so the easiest way is just to alter the two resistors in the mute circuit.
If you change the 33k and 3.3k to a 15k and 12k you would get -7db attenuation instead of -20db. You don't want to load the preamp stage with to low a total resistive value but equally for best results you don't want it to high (although no damage would occur whatever values you pick).
That should make quite a difference and you can get an idea whether you want more or less attenuation. Remember modern signal; sources like CD etc are around ten times the basic output level of old DIN type stuff and still a lot higher than things like old cassette decks and tuners etc.
If you change the 33k and 3.3k to a 15k and 12k you would get -7db attenuation instead of -20db. You don't want to load the preamp stage with to low a total resistive value but equally for best results you don't want it to high (although no damage would occur whatever values you pick).
That should make quite a difference and you can get an idea whether you want more or less attenuation. Remember modern signal; sources like CD etc are around ten times the basic output level of old DIN type stuff and still a lot higher than things like old cassette decks and tuners etc.
Modifying the mute button sounds like a great idea! Could I just replace the 33k with a 4.3k to get the -7db attenuation? The thinking being that the mute button would then give the option between stock and -7db.
Learning what would be involved in altering the gain of the two transistor line stage would be very interesting. Keeping the original mute feature would also be appealing.
Learning what would be involved in altering the gain of the two transistor line stage would be very interesting. Keeping the original mute feature would also be appealing.
You could do that. The only downside is the loading on the output of the gain stage although it still delivers 8v peak/peak driving those values which is more than enough.
Lowering R223 (15k feedback resistor) lowers the stage gain and should also lower the intrinsic noise. You can go as low as around 4k7. You also have the option of trying a combination of the two.
Lowering R223 (15k feedback resistor) lowers the stage gain and should also lower the intrinsic noise. You can go as low as around 4k7. You also have the option of trying a combination of the two.
I agree with Mooly's critique of gain distribution.
I think it should be possible to reposition the balance and volume control circuits so that they come after the tone control and 21.5dB amplifier. More explicitly, the circuitry and controls lying between board pin 1 and the volume wiper (and similar points in the other channel) would be reconnected electrically to be inserted between the board pin 8 and the preamp out jack on the rear panel.
You could preview the modification by installing an external volume control at the preamp output to main amp input links. You'd set amp front gain to max. If you're happy with results, then you'd then do actual modifications.
I think it should be possible to reposition the balance and volume control circuits so that they come after the tone control and 21.5dB amplifier. More explicitly, the circuitry and controls lying between board pin 1 and the volume wiper (and similar points in the other channel) would be reconnected electrically to be inserted between the board pin 8 and the preamp out jack on the rear panel.
You could preview the modification by installing an external volume control at the preamp output to main amp input links. You'd set amp front gain to max. If you're happy with results, then you'd then do actual modifications.
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I tried lowering the feedback resistor to 7k, but didn't notice much difference. I then tried changing the mute resistors, which has solved the issue. The amp is still plenty loud enough with the mute engaged, just without the annoying hiss. I also like the option of being able to go back to the original setup.
The idea of moving the balance and volume is very interesting, but I don't think I have the expertise to carry that modification out at the moment.
Thanks again for all of the help!
The idea of moving the balance and volume is very interesting, but I don't think I have the expertise to carry that modification out at the moment.
Thanks again for all of the help!
Different approaches 🙂
I'm pursuing altering the line stage gain as something easy to try.
This shows the original configuration on the left and the altered on the right.
We have voltage into both in green, the output of the original in blue and the output of the modified stage in red. All that is altered is R223. I've actually gone down to 3k9 here.
If we look at the original version and now alter the mute resistor and the fitting of a 4.3k to give a -7db attenuation we hit the problem of the loading becoming an issue and causing clipping.
The altered gain stage on the right does not clip and offers almost the same attenuation as the -7db mod.
Response of two version, original and modified. This shows the gain down from 21dB to 11dB. The high frequency response is a little more extended as we have reduced the gain.
Square wave response at 20kHz. All good. As before, Vin in green, original output in blue and modified in red.
This is a modification that would be super easy to try because all you need do is tag a new resistor across the original 15k to get the value you wish. A 5.6k would get you to around 4k in total.
Simulation file for anyone interested.
I'm pursuing altering the line stage gain as something easy to try.
This shows the original configuration on the left and the altered on the right.
We have voltage into both in green, the output of the original in blue and the output of the modified stage in red. All that is altered is R223. I've actually gone down to 3k9 here.
If we look at the original version and now alter the mute resistor and the fitting of a 4.3k to give a -7db attenuation we hit the problem of the loading becoming an issue and causing clipping.
The altered gain stage on the right does not clip and offers almost the same attenuation as the -7db mod.
Response of two version, original and modified. This shows the gain down from 21dB to 11dB. The high frequency response is a little more extended as we have reduced the gain.
Square wave response at 20kHz. All good. As before, Vin in green, original output in blue and modified in red.
This is a modification that would be super easy to try because all you need do is tag a new resistor across the original 15k to get the value you wish. A 5.6k would get you to around 4k in total.
Simulation file for anyone interested.
Wow, that's amazing! Well, since you've gone to so much effort I need to give that a try!
I'll let you know how I get on.
I'll let you know how I get on.