HeadphoneMixer Schemo Review

[Samuel Groner]

Hi
Still doing some work on my simple headphone amp... Latest schemo:

[removed]

Any suggestions? Anything done wrong?

I put R1/R2/R11/R12/R21/R22 in because I have a preamp that has "zero" ouput impedance, so these might damp ringing of the transformer and the RFI filter network a bit.

R7/R17 and R36/R41 are ganged.

R25 will, according to the LL1540 datasheet, improve CMRR.

Input bias network for U3 is overkill, but I can't help!

Thanks, as always!

Samuel

 

[PRR]

> R25 will, according to the LL1540 datasheet, improve CMRR. Input bias network for U3 is overkill, but I can't help!

Looks fine. The only objections I see, you've already answered.

 

[Samuel Groner]

Thanks, PRR!

BTW, is there any point in protecting the ins and outs by zeners (or diodes to supply rails) from overvoltage? I remember having read that bipolar inputs don't like high voltages if they are not powered.

Samuel

 

[PRR]

> is there any point in protecting the ins and outs by zeners (or diodes to supply rails) from overvoltage? I remember having read that bipolar inputs don't like high voltages...

You have to be very careful if you take signal from an UNKNOWN source: telephone line, someone else's strange gear, etc.

In this case: you have enough series input resistance to make blow-up unlikely with any normal source. And you will be careful in use. And you will use sockets, and know how to swap, and these are $2 chips. I would not worry one bit.

> ...if they are not powered.

If you have a signal going to an UNpowered box, and the same signal to recorder, picture the chips as a couple diodes. Up to 0.5V they are open, over 0.7V they are dead-short (nearly). With the resistances you have, it won't short-out the signal, but it could add distortion on a 600Ω line. If you are feeding from a dedicated output, no problem, but if this bridges right on your recorder bus you should keep it powered-up or disconnected.

I just spotted the 26r1 and 100pFd. That's 80MHz! Are you under a TV tower? I'd be thinking more like 470r and 220pFd, or even 1K and 470pFd. I didn't check the LC values, have you checked for ringing?

 

[Samuel Groner]

I just spotted the 26r1 and 100pFd. That's 80MHz! Are you under a TV tower? I'd be thinking more like 470r and 220pFd, or even 1K and 470pFd. I didn't check the LC values, have you checked for ringing?

I took the 2nd order LC filter from Bill Whitlock's article "Grounding and Interfaceing" (Handbook For Sound Engineers, 3rd) and added the Rs and Cs for fun.

Throwing in larger resistors and caps in front of this simple diff amp does not sound right to me because the diff amp will see lower impedance at high freqs and thus compensate the roll off with make-up gain, no? Did not check how much it were, though...

Samuel

 

[Samuel Groner]

I took the 2nd order LC filter from Bill Whitlock's article "Grounding and Interfaceing" (Handbook For Sound Engineers, 3rd) and added the Rs and Cs for fun.

Reading this article again, I realise that he used 220 pF caps and that the bandwith is 50 kHz, which sound a bit too though for me. So I leave the 100 pF in and replace the inductors with smaller ones.

Samuel