DIY Diamond Buffer Preamp

[Puffin]

http://www.ebay.com/itm/Opamp-input-plus-Diamond-Buffer-preamplifier-stereo-w-on-board-regulator-PCB-/220894319973?hash=item336e53d565:m:m-ivXMzvFTEB2FOqdO55f_A&clk_rvr_id=1124555777784&afsrc=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.

[Firebottle]

The difference 6.8 to 7pF would be nothing.

But 7pF is an odd value as it doesn't sit in the E12 range of values, eg 4.7, 5.6, 6.8, 8.2, so are you sure the cap is 7pF?

The circuit shouldn't give bass emphasis or treble cut as stands, as Andrew says.

[Puffin]

All I have to go on is they are marked "7" and RoHS. My gadget to check capacitors needs a 9v battery. Will get one when I go to the doctors in 10 mins!

[RothwellAudio]

"7" is probably some kind of manufacturer's batch number or a code for something other than the capacitance.
Can you post the circuit diagram?

[Puffin]

Here it is. Not a very good copy I am afraid, but that is how it came to me.

http://i73.photobucket.com/albums/i239/saxonsex/Jims%20Audio%20HDAM%20preamplifier_zps8v8bexym.jpg

My PDF version is readable if enlarged. I could add it as a PM if neccessary.

[RothwellAudio]

Sorry, I can't read that properly.

[Puffin]

I could try to sketch it out and photograph it. Is there any particular section or do you need the whole thing?

[RothwellAudio]

The power supply/regulator bit isn't necessary, and we only need to see one channel, so the bit round one op-amp and its associated output transistors is the important bit.

[Puffin]

Hope this works!

http://i73.photobucket.com/albums/i239/saxonsex/WP_20170204_09_42_47_Pro_zps05rqnpis.jpg

[RothwellAudio]

R1 and C1 (1k and 220pf?) form an RF filter. If you have the value of one of them way off you could roll off the treble in the audible band.
R3 sets the gain in conjunction with R5. C2 is meant to compensate for the extra phase shifts that the extra output transistors introduce and keep the circuit stable. Does it say 6.8pF - 100pF? Are they suggesting you just try anything in that range?
Anyway, if the capacitor was way too big it would also roll off treble in the audio band.
The worrying thing about that capacitor is that the circuit could be unstable but with the oscillation in the ultrasonic band.

[Puffin]

Hi Andrew, without the 6.8uf caps all you get is oscillation (a whistling/signal type noise) I used a 7pf ceramic cap (6.8pf is the value screen printed on the board). It may be worth me trying higher values to see if it improves the treble, but without oscillation. Are they suggesting you just try anything in that range? This was an ebay buy with nothing other than the board sent to me and the schematic sent by email!

So far as C1 is concerned I will check the board. Because of the position I cannot now see the value. I will prob have to de-solder. Many thanks for your continued support.
Rob.

[RothwellAudio]

At a pinch it looks like C2 could go up as high as about 1nF (1000pF) so don't be shy about using a higher value than 6.8pF. The "7" printed on it is unlikely to be 7pF, so I have no idea what is in there. Try anything between 10pF and 100pF. I'd go for the 100pF end of the range.
If you have put in something like 6.8nF by mistake it would definitely cause an audible treble roll-off.

[Puffin]

OK, will do.

[Arkless Electronics]

Agreed that the whole diamond buffer part is superfluous. It does appear that they are saying "between 6.8 and 100pF" for that cap.. which is rather worrying... I would try in the 33-68pF range...
Do you have a 'scope to check for oscillation?

[Puffin]

Hi Jez, no I do not have a scope. I will try what both you and Andrew suggest. Thanks for your input.

[Arkless Electronics]

Hi Jez, no I do not have a scope. I will try what both you and Andrew suggest. Thanks for your input.

I would expect 6.8pF to be too low for this application... go with 68pf and you should be in the right ball park. 22,33,47,56,68,82 and 100 is the normal range of available values here BTW. Anything it that range which you have available should be better than 6.8pf but without a scope and a minimum of a fast square wave generator you're pissing in the dark really... On the plus side, overcompensation, with the circuit and range of values here, should result in nothing more than rolling it off at 60KHz rather than say 120 so nothing too much to worry about ;)

[RothwellAudio]

BTW, if you just wanted to use this as a pre-amp you could simply take out those extra transistors and put in a link from R6 to R13. Without the extra transistors there might be no oscillation anyway and it would probably sound better.

[Arkless Electronics]

BTW, if you just wanted to use this as a pre-amp you could simply take out those extra transistors and put in a link from R6 to R13. Without the extra transistors there might be no oscillation anyway and it would probably sound better.

Agreed. Almost certainly no oscillation under those conditions and there is no load that the op amp can't drive fine on its own.

[Puffin]

I will do some tinkering and report back.

[Puffin]

Just to let you good folk know that my wife came down with a chest infection last week and the doc diagnosed Pneumonia yesterday, so will tinker when I can.

[walpurgis]

Just to let you good folk know that my wife came down with a chest infection last week and the doc diagnosed Pneumonia yesterday, so will tinker when I can.

Hope she gets well soon Rob.

My wife had the same last year and was very ill.

[Puffin]

Thanks Geoff, the trouble is she still thinks she is a teenager. She insisted last week on going on a 3 day residential outward bound thingy to look after a class of her primary school children. 6.30am start for the "minders" and no down time really (up at 2am first night to a crying girl and some chucking up!) I told her not go as there was no way I would do it and I am a lot fitter than her. Of course it was damp and pissing down.....and "outward".....means just that!

[Puffin]

Managed to grab some time this morning and changed the ceramics to 33pf. Sounds a hell of a lot better. Have only tested it through headphones, so will see what it is loike in the main system. If I am not happy then I will try 68pf as Jez suggested.