kgst

[chiguy]

I carefully enlarged the transistor tab holes with a drill bit and used 3PPSG.  

 

Correct way to mount transistors to their heatsinks: http://www.aavid.com/product-group/accessories/mounting-kits

[Earspeakers]

Hmm, at $2.50 a pop. At least it gives us the part number for the shoulder washers

 

http://www.mouser.com/Search/ProductDetail.aspx?R=7721-7PPSGvirtualkey53210000virtualkey532-7721-7PPS

 

 

If by isolator you mean the shoulder washer, you want to use 7721-10PPSG. The bushing is 2.41mm which is long enough to extend through the tab of the transistor and some. 7721-3PPSG bushing is 3.81mm but the bushing's outer diameter is slightly larger. In my case, it would not fit through the tab hole of the transistor unless you file and enlarge the tab hole.

The ceramic insulator is either AAVID 4170G or 4171G. Both should work fine. 4171G is thicker but smaller in dimension.

I use 10PPSG with 4171G and the combination works well. Although I may have used 3PPSG if I had the proper file to slightly enlarge the tab hole of the transistors.

Hope this helps.

 

 

What about the 7721-7PPSG? That's what AAVID uses in their TO-220 kit above. 

Edited April 3, 2015 by Earspeakers

[mwl168]

When I pick the shoulder washer, I purposely picked the ones that the bushing is long enough to extend through the entire thickness of the tab and further into the ceramic pad. 

 

Bushing length for 7PPSG is only 0.81 mm according to the datasheet which is not long enough to cover the thickness of the transistor tab. For high voltage application I personally don't fee comfortable using it.

[Earspeakers]

When I pick the shoulder washer, I purposely picked the ones that the bushing is long enough to extend through the entire thickness of the tab and further into the ceramic pad. 

 

 

Thanks, that's perfect. 

 

So I've been running the KGST for a couple of days and am checking bias/balance/offset again. I had previously set these numbers by the procedure above, letting them run an hour or longer and readjusting. 

 

• The headphone bias drooped to around 517V
• One channel balance was around 10 volts and offset was 17, I could zero the balance but can't get the offset to zero
• The other channel had good balance, but the offset is likewise 17 or something, and I can't zero it. 

 

Help! Why would these be just fine when I set them before, then decide to uncalibrated? How can I get them calibrated again? 

[nopants]

change the resistor in series with the offset pot

 

where are you measuring the bias from

 

did you do these adjustments with the box open and then recase the amplifier

[Earspeakers]

change the resistor in series with the offset pot

 

where are you measuring the bias from

 

did you do these adjustments with the box open and then recase the amplifier

 

• The bias is measured from the test point
• The adjustments are all open with the case open, so yes it's warmer with the case closed. But this has pretty good ventilation (see a page back), but maybe that's the problem?

 

Change the resistor in series to try what value? Increase it, decrease it? Looks like 1.2k or 1.37k from 1.5k is the answer. 

 

EDIT: Wuupsies (maybe)

 

Looking closer at the circuit board I see mostly obscured, two little circles on the trims, indicating where the trim adjustment should go. I've got the balance in backwards. But these trims are symmetrical, aren't they so it shouldn't matter? And if they weren't, why did they calibrate so easily when I first turned them on? 

Edited April 3, 2015 by Earspeakers

[Kerry]

It doesn't matter which way you put the pot in.

[Earspeakers]

Well, because I love doing rework so much I swapped out the 1.5k with a 1.21k, fortunately having some on hand. Now it seems I can calibrate again. 

 

Curious, this seems like a common enough problem, why not just make it a 1.2k, or split the difference with 1.37k?

[Laowei]

Because this is DIY?

[Earspeakers]

Well I mean change the legend, for the newbies. 

[nopants]

or read the thread

[Laowei]

The KGST is not a production amp.

Kevin originally designed it for 400VDC some time ago. That's where the 1K5 value resistor came from.

Birgir helped in designing the PCBs, built and debugged the prototype. From that build he and Kevin settled on 350VDC supplies, so the value of that resistor may not be optimum for some builds. Depends on the actual supply voltages, actual CCS settings, actual tubes or any combination.

Since that time Kevin and Birgir have gone on to design more than several new amps for Stax HPs. Let's keep them free and creative out on the cutting edge not having to get into the little BS running changes. Hell, the circuit is 99.5% there.

You are coming to the party late, so don't expect anyone to be tidying up the designs. They are a constant work in progress, given to us to work out for ourselves. Besides you will learn more figuring out the circuits by debugging them. And you can ask away as you have on this forum for help. That is why this is called DIY.

[Earspeakers]

or read the thread

 

Which I did, which is why I'm also making sure to write about it here so follow on people get the news. I'm also working on a FAQ and documentation for these boards, my small contribution. 

 

Kevin originally designed it for 400VDC some time ago. That's where the 1K5 value resistor came from.

Birgir helped in designing the PCBs, built and debugged the prototype. From that build he and Kevin settled on 350VDC supplies, so the value of that resistor may not be optimum for some builds. Depends on the actual supply voltages, actual CCS settings, actual tubes or any combination.

 

Ah that makes sense now. 

 

 

Since that time Kevin and Birgir have gone on to design more than several new amps for Stax HPs. Let's keep them free and creative out on the cutting edge not having to get into the little BS running changes. Hell, the circuit is 99.5% there.

You are coming to the party late, so don't expect anyone to be tidying up the designs. They are a constant work in progress, given to us to work out for ourselves. Besides you will learn more figuring out the circuits by debugging them. And you can ask away as you have on this forum for help. That is why this is called DIY.

 

Don't get up in arms, but just to note, Birgir has been tweaking the KGSSHV. Not long ago I happened to buy one of his (one of the last) with the feedback, shortly thereafter he removed it, that's certainly a bigger change then a little text. Also don't get the wrong impression here, I'm not complaining. The reason I mention it is if the community would like to avoid newbie builder issues in the future (e.g. constant questions about the resistor) the easiest thing would be to change the writing. Just saying!

[spritzer]

It has already been changed on the new boards:

 

 

Been like that for months. 

 

There is a good and a fairly obvious reason why we did it like this though. 

[Earspeakers]

It has already been changed on the new boards: Been like that for months. 

 

Might check that Kevins site has the latest, I just did a board run not long ago and it was still 1.5k. 

 

In fact I can see the PS board is quite different. Looks like it's the "mini" version. 

 

Amp boards look to be unchanged other than the 1.2k. 

Edited April 4, 2015 by Earspeakers

[spritzer]

The amp board is different.  It's a bit smaller to suit my needs. 

[Earspeakers]

Now I have a moral dilemma, since I'm building every mafia design I can get my hands on (and documenting them for posterity), should I also build out the revisions??? I actually do have a second KGST transformer, and a smaller one wouldn't hurt .... must stop ...

[Earspeakers]

Back to these amazing amps, what's really weird all the traffic noise I hear. I only listen to classical music, a lot of concert hall recordings (or almost all). I've had this happen once before with some horns, but now even more I'm getting major traffic noise from the recordings. Especially the older halls are prone to this. I keep having to pause just to make sure it's not in my environment (I'm in a very quiet room on the edge of town (almost the country). 

 

Truly amazing little amps, congratulations to the Mafia and community. 

[insanity]

I also sometimes hear traffic noise in some older recordings. Thats the way they are and maybe the producers did not even hear it with their equipment...

[spritzer]

Now I have a moral dilemma, since I'm building every mafia design I can get my hands on (and documenting them for posterity), should I also build out the revisions??? I actually do have a second KGST transformer, and a smaller one wouldn't hurt .... must stop ...

 

If you are really bored as there are hundreds of revisions, some tiny but also some major ones. 

[Earspeakers]

These are newer recordings. I used to do recording engineering (non professional), with big groups you can't avoid miking the hall so pick up on the total ambiance. Davies Sym hall in SF (where I played for years) is a double wall construction. To get inside you go through thick double doors, the outer hall is essentially a shell. I would sometimes go out on stage and it would be almost dead silent (a tiny bit of low frequency air conditioning that was impossible to entirely remove.) In the middle of San Francisco, it was magical. 

 

 

The most famous for this is the Opera house in Rome, name is slipping my mind at the moment, you can hear Vespas zipping around the plaza outside. The Callas recordings are known for this. 

 

If you are really bored as there are hundreds of revisions, some tiny but also some major ones. 

 

 

No sir! KGSSHV next couple weeks, then climbing Megatron Mountain, and finally summiting T2.

[headinclouds]

Back to these amazing amps, what's really weird all the traffic noise I hear. I only listen to classical music, a lot of concert hall recordings (or almost all). I've had this happen once before with some horns, but now even more I'm getting major traffic noise from the recordings. Especially the older halls are prone to this. I keep having to pause just to make sure it's not in my environment (I'm in a very quiet room on the edge of town (almost the country). 

 

Truly amazing little amps, congratulations to the Mafia and community. 

Yes that's true, I do hear a few noises that were never audible on vinyl.  Reminds me of some reviews way back of say Decca recordings made in London they mentioned the rumble of tube trains from below.

[eggil]

You guys then need to listen to Glenn Gould. His humming is unmistakable and easy to follow with the Megatron and the 009

Edited April 5, 2015 by eggil

[insanity]

Definitely true for gould. I  recently attended a concert of maurizio pollini and he was humming or singing all the time too .

[Earspeakers]

Yeah I've got the complete Gould. I understand the engineers tried their best to edit out his humming, I wish they hadn't bothered. Adds to the performance IMO. This is the later Gould, the early version bores me to distraction. 

 

Funny to hear new stage noises in recordings I've listened to for years. "What are they doing back there?" I keep catching myself thinking. 

Edited April 5, 2015 by Earspeakers