AR SP8 mk2 high output volume

AR output stage circuit diagram. https://ibb.co/Chrcx4t

OMG- while you are at it, jumper out R-30, C25 and C26. This will allow the feedback to include everything that occurs at the output of the preamp. I would also consider eliminating the C24 bypass cap and replacing C10 with a good quality part of the specified value (bypass caps tend to smear the signal when bypassing a larger coupling cap). D18 really doesn't protect the tube during warmup, but it does affect the sound so removing it is a pretty good idea. ARC left a lot of performance on the table in this design...

BTW this preamp is old enough that the power supply filter capacitors should be replaced if you want it to be reliable. If not replaced, the preamp could develop a hum, but could also suffer a power transformer failure and that is an expensive part to replace if you can even find it (ARC does not stock it anymore- they purged older parts like this years ago).

Good to know the A30 volume pot does not degrade the signal when set lower than full. I learned something.

@audite84  The volume control is a 100K device. Combined with the input capacitance of the voltage amplifier tubes (which are essentially 6SN7s) there will be a high frequency rolloff. This is called Miller Effect. But in this case the values have been chosen well so that the rolloff is occurring well outside the audio band. The worst area of rolloff is typically around the middle of the control range; on paper at least its affecting things around 90KHz and above.

Quite often George and I disagree, but not here.

@mrdecibel  This 'integrated' amp does not employ a separate gain stage for a preamp section. Its a power amp circuit (a variant of the Williamson circuit) and simply has enough gain that any source with a maximum output of 1 volt (such as an old school cassette deck) can easily drive it to full output. 

when I put the AR I notice a plus in terms of presence and forcefulness of the music

OK. I asked simply because with a high level signal like a CD player I would not have expected adding additional circuitry would have helped out.

@tenoris

Having looked at the schematic supplied by @imhififan I agree that essentially this amplifier is actually a power amplifier supplied with three inputs and a passive volume control.


Since the control is 100K and given the characteristics of the input tube, you can successfully operate the volume control to reduce the level of signal coming from the ARC without degradation.

However if the sources are high level (in other words not phono), I am still having trouble seeing what good the ARC can do for you unless the sources are not physically fairly close to the amplifier. It seems that you are saying that it nevertheless sounds better this way- so I am curious, what improvements are you hearing?

BTW I do not recommend tampering with the feedback resistor- such things can lead to oscillation, and if the schematic is correct there appears to be little or no compensation for a variety of frequency poles present in the amplifier design- by increasing feedback, any one of these poles could cause the amp to be unstable.

I am using the Puresound A30 as a power amp because it is a possible option and so indicated by the manufacturer.

I have just tried to use the Puresound attenuation as you have indicated and the result is quite satisfactory, with this volume at 1:00, I can have a tour in the AR between 10 and 11 as a high volume level

@tenoris
If the above two statements are correct then you might be using the wrong input- use the one marked 'amplifier' rather than something like 'AUX'.


If this amp can be used as a power amp it will have an amplifier input, and that will reduce the gain such that the ARC can be easily used.


If it does not have an amplifier input, then what the manufacturer told you is incorrect, and there really isn't any advantage I can see from using the ARC- it should sound better just running the amp by itself with its own volume control.

@tenoris
Why are you using a preamp? Your amplifier is an integrated amp.

If its for the phono, use the tape output of the ARC instead of the main out- problem solved.