Tape Repro Amp

[RSRecords]

Ok originally, I had it like this then mistakenly changed it. Sim looks pretty good on this with the VRhi changing the breakover point.
However You are saying that this would be unstable? I can't get the sim to work right with

My understanding is that the result is not so much a lift but a leveling out via a sort of hi pass filter. The more I study about filters the more I realize I how poor my math is. Goal was to make it very simple and easily switchable to other standard eqs.

 

[RSRecords]

As drawn the circuit has little gain simply because of the 10K resistor from the inverting input to ground but I guess the prior stage provides the required head gain.

Yes the goal was to be able to switch between similar eq standards with minimal gain change. The bulk of the gain will be set by the input amp (1512).

If I can land on a topology experimenting with the values is easy enough. Though making things variable also creates some challenges.

 

[ruffrecords]

Yes the goal was to be able to switch between similar eq standards with minimal gain change. The bulk of the gain will be set by the input amp (1512).

You will need to watch the headroom of the first amp because the rising head output voltage could easily span a 40dB range

If I can land on a topology experimenting with the values is easy enough. Though making things variable also creates some challenges.

IC1A still has no dc connection from the output to the inverting input. A 1M resistor should do the rick. Spice ac simulations don't care about the dc conditions so they will work anyway even if the real circuit will not.

Cheers

Ian

 

[abbey road d enfer]

If this is a replay amp with a 3150Hz turnover then we need a flat response above 3150 and an LF lift below it.

Exactly. Since the response due to R1 & C1 is already a 6dB/octave low-pass, the response of the subsequent stage must be a HF lift.
That could be achieved by putting a resistor in series with C1.

 

[ruffrecords]

Exactly. Since the response due to R1 & C1 is already a 6dB/octave low-pass, the response of the subsequent stage must be a HF lift.
That could be achieved by putting a resistor in series with C1.

Now I am really confused. So what is the purpose of R1 and C1 in a tape replay amp?? Is that meant to be the LF lift, in which case don't you just flatten the response at 3150Hz with a resistor in series with C1?

Cheers

Ian

 

[RSRecords]

Having trouble getting a value of that bottom resistor that makes sense. Maybe there is different approach I should be taking. The 50Hz pole is easy enough but I can't work out an easy way to level off the high end at a particular frequency and maintain particular gain (at 400Hz.)
6k, 470nF, and 100R seem to be spot on.

 

[ruffrecords]

The network completely defines the response. Your very low frequency response is effectively unity and once the HF levels off the loss is determined by the 6K and the 100 ohms i.e. (6000 + 100)100 = 35.7dB. The LF boost is close to 20dB/decade so at 315Hz the loss will be about 15.7dB so maybe about 16dB at 400Hz.

The easiest way to obtain an exact figure is to simulate it.. The loss at 400Hz will depend on the network so you may need to sim it for each curve. And to be really accurate you heed to account for the source and load impedances.

Cheers

ian