DIY Diamond Buffer Preamp

[Puffin]

http://www.ebay.com/itm/Opamp-input-...c=1&rmvSB=true

I bought one of these in 2010, built the board up and did nothing else with it I recently made it up and it works......but IMO not that well. There is a lot of LF and not a lot of HF coming out. I measured the Ohms out and (if I did it correctly - one probe to signal out + and the other to -) and I am getting 100ohms. I emailed Jims Audio and he said that this was correct. The board can be used as a H/P amp as well as a Preamp. I would have thought that the output should be around1k?

I have been testing it using a simple H/P amp (op-amp based).

I have the schematic.

[RothwellAudio]

1) What do you mean by "measuring the ohms out"? If you mean you measured the output impedance, you can't do that with just a multimeter.
2) How would something with a 1k output impedance drive a pair of 32 ohm headphones?
3) What is your question?

[Puffin]

Hello Andrew,

1) As you will have twigged, I don't know what I am talking about. I qualified my "reading" of the output impedance and you have confirmed that it is nonsense.
2) I suspect that the answer to this is that it can't.
3) Is 100 Ohms correct.

Thanks for taking the time to reply. Rob.

[Firebottle]

Have you fitted the correct value of cap on either side if the op-amp.
Should be 6.8pF looking at the board.

[RothwellAudio]

Yes, 100 ohms seems like the sort of reading I would expect if you were to simply measure the resistance at the output terminals.

The circuit shown on the ebay listing is a basic op-amp with added output transistors to allow the circuit to drive low impedance loads (such as 32 ohm headphones). Feedback is taken from the output so the output transistors are in the feedback loop. You have to be careful when you put extra transistors in the feedback loop to avoid instability. If it's working ok it should work without any bass emphasis. If it isn't it might be difficult to identify the fault via forum exchanges.

Was your intention to use this as a headphone amp or as a pre-amp?

[Puffin]

Have you fitted the correct value of cap on either side if the op-amp.
Should be 6.8pF looking at the board.

Hi Alan, the nearest I could get was 7pf. Could this be the problem?

[Puffin]

Yes, 100 ohms seems like the sort of reading I would expect if you were to simply measure the resistance at the output terminals.

The circuit shown on the ebay listing is a basic op-amp with added output transistors to allow the circuit to drive low impedance loads (such as 32 ohm headphones). Feedback is taken from the output so the output transistors are in the feedback loop. You have to be careful when you put extra transistors in the feedback loop to avoid instability. If it's working ok it should work without any bass emphasis. If it isn't it might be difficult to identify the fault via forum exchanges.

Was your intention to use this as a headphone amp or as a pre-amp?

Andrew, I intend to use it as a pre-amp. Please also see my answer to Alan's question.

[RothwellAudio]

If it's a pre-amp you want I would probably do away with the extra output transistors and just use the op-amp part of the circuit. Well actually, I wouldn't - I would make a simple op-amp pre-amp on a piece of perf board. Anyway, the extra transistors are probably just compromising the performance of the op-amp alone and aren't needed if you don't want to drive headphones.
The 6.8pF capacitor makes me suspicious. Very small values of capacitor suggest to me that the circuit might be unstable and the capacitor has been added to try to kill some oscillation. That's just a guess. A look at the circuit diagram would help.

[Puffin]

If it's a pre-amp you want I would probably do away with the extra output transistors and just use the op-amp part of the circuit. Well actually, I wouldn't - I would make a simple op-amp pre-amp on a piece of perf board. Anyway, the extra transistors are probably just compromising the performance of the op-amp alone and aren't needed if you don't want to drive headphones.
The 6.8pF capacitor makes me suspicious. Very small values of capacitor suggest to me that the circuit might be unstable and the capacitor has been added to try to kill some oscillation. That's just a guess. A look at the circuit diagram would help.

Yes, you could be right. As I couldn't get 6.8pf, I intially tried it without them. All I got was variable whistling/signal type noise as I turned the pot. It varied in intensity with the pot position. I am actually using a simple op-amp pre at the moment, but then remembered that I had this that I had never used. Bad idea?

[dimkasta]

I also do not see an output capacitor. Make sure you have no DC on your output.

I would have to agree with Andrew.
Keep the regulators if you want, and just build something simple like an opamp buffer or a simple JFET follower. Really easy to breadboard.