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Old 16th March 2012, 09:48 PM   #1
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Default Spare bits preamp

Hi All,

Thought I'd share my latest meddlings.
Basically, I have been feeding my JLH 96 with a DAC via a passive pre. My passive pre has been either a linear 10k potentiometer, or a nice Alps 50k potentiometer. The linear is the right kind of impedance, but cannot be used quietly because of it's taper, and the 50k alps is lovely, but seemed to sound lifeless.
So, I thought I'd try and build an active buffer stage for my Alps pot, and call it a preamp
I set myself the rule that it needed to be done completely free of charge - using only bits I had lying around. Thankfully I did have some nice bits and bobs, although some bits are a bit overkill (120VA 15-0-15 toroid, 2x10,000uF 63v Cerafines), and some possibly not good enough (317/337).

The design is based on one of the Naim preamp schematics found on acoustica.org.uk (if you have a look at the Naim preamp tweaking page...). I used a similar design as a headphone amplifier years ago and wanted to see how it coped as a line level buffer.
I wanted to try a DC coupled B1 buffer, but thought it might be fun to try something with BJTs (and I have very few JFETs and loads of BC550s {and thankfully 2 BC560s!} - so able to keep the cost to zero).

I wasn't sure how much of a difference the circuit would make. Frankly, having listened to it for a while now, I am frankly gobsmacked! It sounds awesome. Suddenly, because the DAC is no longer struggling to drive the input of the amp (because the amp saw a much larger source impedance when I used a passive preamp) bass seems far more prominent, and detail in general drastically improved.

I've attached the schematic. The only issue I've had is that, without the 100nF capacitor across the output, I get a lot of hash, which is odd. So I suspect I'm getting some parasitic oscillations somehow. Perhaps I need to add some very local decoupling capacitors to the circuit. Despite the knowledge of potential oscillation, albeit filtered out, it sounds great!
I wanted to attach some pictures, but I've just realised they are too big to post, so the schematic will have to do for today.

Next, I want to build an input select (I have some reed relays kicking about), and a JLH headphone amp. It might get a bit tight finding all the bits for the headphone amp without spending anything, but I'll give it a go.

I'm certain this build isn't perfect. There have been some interesting compromises. But it's been great fun to do!

More pictures to follow.

Cheers,
Phil
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Old 17th March 2012, 01:09 AM   #2
djoffe is offline djoffe  United States
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I"m guessing that you're oscillating. Maybe 100 pF from collector to base of your PNP output transistor would tame it. What do you think?
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Old 17th March 2012, 09:37 AM   #3
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Yes, I was suspecting something like that, but unsure as to where to look. I think I have something similar in size. I'll give it a try. Thanks for the suggestion.
I'll also try and downsize my photos of it
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Old 17th March 2012, 11:26 AM   #4
godfrey is offline godfrey  South Africa
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Is that 100nF or 100pF at the output? 100nF would roll off the treble above about 5KHz.
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Old 17th March 2012, 05:23 PM   #5
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Hi Godfrey,

It's 100nF. I thought with the 300 ohms, it forms a 1st order low pass filter with a 3dB point of about 33kHz.
Sounds as if it's reproducing the highs okay to me. Cymbals sound great. But I'll look into that.
Besides, it's not ideal having it there. Wasn't intended. I had to add it to filter out some peculiar noise.
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Old 17th March 2012, 06:58 PM   #6
djoffe is offline djoffe  United States
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I did a quick sim...I shows the peaking I expected, indicative of problems with oscillation. Pictures show response with compensation cap Cc at 0 pF (green) and 47 pf (blue). Response plotted is from input base (bin) to output collector (cout). You can probably remove the 100 nF on the right side of the 300 Ohm resistor.
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File Type: png bitsampckt.png (44.6 KB, 229 views)
File Type: png bitsampresponse.png (59.0 KB, 206 views)

Last edited by djoffe; 17th March 2012 at 06:59 PM. Reason: more clarity
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Old 18th March 2012, 12:01 AM   #7
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Wow!
Thanks for that. I've been trying to simulate this, but found the latest Eagle (which I've used in the past) doesn't have a simulator (or I couldn't find it at least!). I shall have a go next week. I agree, I would like to lose the 100nF.
Once I've resized the photos so they're small enough files for the site, I'll post them.
Thanks again!
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Old 18th March 2012, 12:48 AM   #8
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Hi Phil - Glad you're still playing!

Other things to try which may not appear related , but might help baseline quality:

1) Try splitting the 47K resistor into 22K+22K with the 100uF cap to 0v from the junction of the two resistors. It will make the Q1/Q2 CCS quieter and more stable. If you want to add to that, a small resistor in the collector of Q1 (say 100R) stops the output CFP 'seeing' an capacitative load in the CCS. This can help HF happiness.
2) djoffe has posted on compensation caps for the Sziklai output, well worth experimenting with. But an alternative approach is to intrinsically damp things by adding a small resistor in the base connection of Q13. Try, say, 47ohms or 100R or even 220R; it can be enough to attack the same root cause at source.

That 33oR/0.1uF output filter is currently a bandaid at best - shouldn't be needed. If you're still nearby please pop over for coffee and the o'scope!

Best
Martin
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Old 18th March 2012, 11:31 PM   #9
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Hi Martin,

Good to hear from you! In fact, the output caps were once yours (this really is a project where I'm determined to reuse old parts ).
Yes I am still nearby. Meeting up sounds like a plan some day. I have a JLH worth a listen to as well.

I see what you mean about the base resistor. That's what I tried with the input NPN. The base resistor there was originally 820 ohms - hoping to stall any oscillations, and after comparing to the Naim schematic, decided to up it to 2.7k (which I had to hand). Seemed to improve things a bit, but not as much as the bandaid.
Thanks to all for the tweak suggestions. I shall have a play this week (and I'll downsize some photos too!).

Phil
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Old 19th March 2012, 10:04 PM   #10
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Don't you just hate it when you've written a whole reply, then your wireless drops out, and when it's finally sorted, you have to rewrite it!?

Here's some resized photos. Rubbish, because I used my phone. One day, I'll use a real camera and post a gallery of all my heath-robinson-esque kit.

So, first a photo of the buffers being made. I wasn't sure how long they'd be, so I cut them to length afterwards.

Then a shot of them loosely in place, using the PSU just to get it working. Recognise those tants Martin?
Then, mounted in situ. Much neater now, but I will dismantle them later in the week to try to stifle that oscillation.

And, a shot from the back. There are extra holes for more connections. I hope to add an input select when I finally get round to it.

The toroid, and reservoir caps are possibly overkill (although very lovely to have - nice and heavy too), but I had them to hand, and had little else use for them, so could justify their use.
The case is an old plywood shipping crate that was thrown out by my old employer. A bit of sanding and some danish oil, and voila, perhaps no longer some old shipping crate - ok maybe not.

And, of course the lovely Alps pot, niftily fixed with a spare cable tie - no expense spared.
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File Type: jpg DSC00253-small.jpg (70.8 KB, 54 views)
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