Can you explain? Phono amp input stage

[rsrphotos]

I just installed a Rotel RC-995 preamp and got the service manual from Rotel to explore the circuit topology.

The phone stage sounds very nice and is not at all the usual RIAA preamp. It has two gain stages. The input stages is flat, and provides about 15/35 db gain for MM/MC. This is described further below. This is followed by a passive HP filter at 2.1kHz. The output stage is an NE5534 opamp used as a non-inverting amplifier with about 20dB gain at 1kHz. The 50Hz HP and 500Hz LP poles are achieved in the feedback loop.

Attached is a pdf of the input stage. It is a differential amplifier that uses an opamp in the feedback loop. Can anyone explain this circuit? I don't quite get it. The output is taken from the output of the opamp.

I am also surprised to see the bass boost done in the output stage. Most circuits I have seen do this first before the 2.1kHz rolloff. What are the advantages/disadvantages of this ordering?


Richard

[Nelson Pass]

It's not particularly unusual. Many MC preamps have a discrete
low noise input stage, and because the RIAA equalization
provides quite high input signal at high frequencies, there is
an opportunity to do some of the HF roll-off at it's output.

Discrete devices as the front end for an op amp have also been
around for quite a while, with the discretes giving the input
noise figure you want and the op amp giving the high open loop
gain, desired in this case because 60 dB or more at 100 Hz
requires lots of gain.

[rsrphotos]

Thanks, I see now. I was looking at it backwards, the opamp as a buffer for the differential amp. It makes more sense that the differential amp is the buffer for the op-amp gain stage. Incidentally Rotel uses a TL071 here, not that indicated in my CircuitMaker shematic.

BTW, where did my pdf go? I attached it to the original post but it does not show in the thread. Here it is again.

[janneman]

Originally posted by rsrphotos
[snip]BTW, where did my pdf go? I attached it to the original post but it does not show in the thread. Here it is again.

....copyright..?

Jan Didden

[Jonathan Bright]

In response to one point you make. If you have passive top cut for the 2.1Khz TC between gain blocks that does increase the headroom for subsequent stages and is also one way that may reduce the chances of TIM as the signal coming off the record has a rising amplitude with frequency, it also allows you to run lower supply rail voltage (because of the lowere sig' levels). But you have opened a large can of worms as there are a huge range of opinions on how to balance the various requirements in phono' pre-amps.

[Quattor]

I'm really interested in Rotel's schematic. Could you please send it to me?! My e-mail address is quattor2@suonline.net

Thank you very much!

Nick

[rsrphotos]

I'm really interested in Rotel's schematic. Could you please send it to me?!

Well, I only have a paper copy of the Technical manual that I obtained directly from Rotel. I don't think it is in my right to reproduce and distribute it. Contact Rotel tech support, they are very helpful and accomodating and am sure they would supply you with any schematics you want, in particular for out of production models.

As for the comment above "...copyright?" with respect to the circuit snippet I did post, was I wrong to have attributed this to the commercial schematic I extracted it from? What are peoples feelings about that here?