Revox PR99 input PCB

Hi,

I am trying to figure out what gains both stages are running at.  The first opamp driven with transformer matching impedance has inverting -IN directly coupled to the transformer.  Transformer is 1:0.62 IC is dual RC4559.  It is single supply +20V circuit.  Reference voltage of 10V is applied to inverting (direct) and non-inverting input via 100ohm resistor, and than passed to second stage over 2k2 resistor after 47uF blocking DC coupling cap.  DCR of the secondary is ~120Ohm with primary open so that would make sense from CM and bias perspective to keep both inputs impedancies close.  Maybe I am 100% wrong about how it works.

I am lost at the assumption that expected impedance of the connected to the primary side external source impedance (output from mixer) is in the range of 80Ohm so it should be seen from secondary of T1 as 80/0.62^2 = 200Ohm.  That would give IC1 gain of  2400/200=11. 

I don't know how to treat RF filter on the primary.  Does it have additional impact on the impedance of the sec side?  It is low-pass filter with Zobel network to fix time response/ringing and HF noise as I understand it.  I need some help in explaining reflecting impedances process I guess.

The second stage has adjustable gain within 0.2 to 1.5 and when deck is calibrated it is just gain=~1.
Do I undertsnd it corectly?


Best
Andrew
 

This circuit is a zero-field input (in which the xfmr acts as a current xfmr and sees very little voltage, which is beneficial regarding saturation and THD).
If you sim the circuit, you will see that the xfmr sees almost no voltage and the output of the 1st opamp is 9dB below input. The second stage with RA101 in mid position would introduce another 4dB attenuation. IIRC, the nominal input level of the PR99 is +6dBu, which would yield -7dBu at the input of the record amp, which seems to be consistent with the B77, from which the PR99 is derived.
Positive FB on the input opamp helps extending LF response. Very clever...

The 1 M resistor is there to prevent that the output of the OpAmp will hit one of the supply rail voltages when the wiper of RA101 doesn't make proper contact. In this case the gain of the OpAmp is 1x.

Cyranodb said:

Abbey road,

It is too clever, I missed the whole concept, getting to read more about zero-field..

Zero-field - (patent) - An input transformer circuit in which transformer-isolated, balanced inputs are provided operates with low distortion and employs a transformer that has an equal number of turns in the primary and secondary windings

Click to expand...

equal number of turns is not a requisite at all

and which is operated in the current mode to drive the two inputs of an operational amplifier.
One input of the operational amplifier is connected to ground and the circuit operates to drive the other input to virtual ground, so that by transformer action the signal voltage across the ends of the primary winding is essentially zero. Because the signal voltages across the primary and secondary windings of the transformer are both substantially zero, there is relatively no induced electromotive force and no signal distortion occurs in the transformer.

http://www.lundahl.se/pdfs/datash/6404.pdf

To confirm PR99 proper input board levels should be set for 1.55V  and the first stage drops voltage 2.8 times while the following stage can increase by ~ 1.5 times for final record stage delivery at nominal plus ~0.3V.

>>Unity gain stable opamp would be a must with such a low gain?

Click to expand...

In fact there is an actual significant voltage gain between the output and the voltage delivered by the xfmr, so stability is rarely an issue.

The next stage, in mid (1/2*4k7) position, do I add 1/2*4k7 to 3k2 input and to 1k to calc the gain? 

Click to expand...

Yes, that's how I came to the figure I gave you.

1M does little to the final numbers.  Why is it there?

Click to expand...

As RuudNL said...

Cyranodb said:

Thank you guys for valuable feedback, I have problems with this board and I measured Vout from the first stage and this was as you said lower than input, that made me focus on that IC. 

Regarding the gain, I have also later realized that with trafo/IC interaction making differential voltage across INputs by the rule so small calls for substantial gain.

But I am wondering how does it really work.  Can I look at that circuit drawn that way???

Click to expand...

Yes, correct.

  I am thinking now that output cap 47uF HAS TO be polar, as the potential difference across its legs would make the electrons flow through the coil in one direction, if not they would have to sum and cancel on the virtual ground cap. 

Click to expand...

I'm not so sure about it. The output of the opamp should be at the same voltage as the input (+/- some offset)

What happens then to the bias current, does the voltage divider cap source it or sink it? 

Click to expand...

The resistors in the divider will absorb it. Whatever polarity it is, one or the other will take it.