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Old 20th February 2011, 02:58 PM   #1451
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If you take a look at the datasheets, you'd see that it takes indeed close to 400 ohm to reach that bias level (for average devices)
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Old 20th February 2011, 03:00 PM   #1452
Liliya is offline Liliya  Norway
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Here is a photo of the power unit I use for the front end stage to my Goldmund clone.

The idea how to connect another transformer to the one I already have, is taken from an earlier built I did some years ago: LFA 50 class A amplifier.
I use a 2x20 V AC transformer to each channel to get 2x80 V DC out to the front ends(and to stand lower than 100V DC).

You can find more info here how to connect the transformer:
http://www.jls-info.com/julien/audio...a_Alim_LFA.gif

I have dismounted the original power to the front end. Many of this components can be re-used into MJL 21193s power unit.

Eivind Stillingen
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Old 20th February 2011, 04:00 PM   #1453
Liliya is offline Liliya  Norway
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jacco_vermeulen, you wrote in # 1174:

In case you run into stability issues, ground the heatsink and solder the C's at the drains to the heatsink. (in the Hitachi appl. note, btw).

From the heatsinks: Do you mean 1) to ground on the PCB or 2) to the point where I ground the "hole" amplifier to the chassis (in my case, by the phone socket for input)?

Solder C to the heatsinks:
If you look at Alex PCB and how C are connected to the drains, how can this be done in a practical way? They are now connected between Drain and Source.

I consider to use both this recommendations from you, just in case.

Eivind Stillingen

Last edited by Liliya; 20th February 2011 at 04:03 PM.
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Old 22nd February 2011, 08:29 PM   #1454
Liliya is offline Liliya  Norway
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With good help from MJL 21193 to do some component changes to his power unit so it fit my needs, I have now finished the work. With this power unit I am able to ajust the front end voltage to exactly +/- 80 V DC.
This also stabilize DC offset on speaker out.

MJL have made some measurement. This unit, I think, outperforms by a wide margin the original. You can read about the unit here: Patchwork 6: Power Supply Upgrade

After this changes, I am looking forward to listen to the Goldmund later to night

Eivind Stillingen
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Old 22nd February 2011, 08:59 PM   #1455
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Quote:
Originally Posted by Liliya View Post
With good help from MJL 21193 to do some component changes to his power unit so it fit my needs, I have now finished the work. With this power unit I am able to ajust the front end voltage to exactly +/- 80 V DC.
This also stabilize DC offset on speaker out.

MJL have made some measurement. This unit, I think, outperforms by a wide margin the original. You can read about the unit here: Patchwork 6: Power Supply Upgrade

After this changes, I am looking forward to listen to the Goldmund later to night

Eivind Stillingen
But of course , I commented on the infantile voltage doubler/cap mult. from the very beginning. Anything would outperform the "profit margin influenced" original design. A GOLDMUND deserves far better.

OS
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Old 23rd February 2011, 08:07 AM   #1456
Liliya is offline Liliya  Norway
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Ostripper

Yes, you are (were) absolutly right. I took your "warning" from the first time you wrote about this weakness, and I have been looking around to find a suitable power to handle such high voltage.

Last night I was listening to my Golmund and I must say again: The way this amplifier play the upper octave, has never been done in my house before (and I have own and listen to a lots of gear during the last fourty years). This was also my main reason to jump into this risky procject. "The very best" does not excist (in advertising,yes) but different amplifier have different strenght and weakness. I have read about this amplifier before building it (The Absolute Sound among others) and the oppertunity to own a Goldmund within a reasonible price (yes, I agree, all Goldmund equipment are heavy overpriced) challenged me. The Goldmund clone will soon "take over" the place to handle frequencies from 600 Hz and up to the top in my Magnepan 1.6. I run this loadspeakers from a heavily modded DCX 2496 trough NAD 208 (subwoofer), a Nelson Pass F4 (50-600 Hz) and a modded MA tube amplifier for the top.

Talking about stability and how this amplifier measure, there is still a job to do. I will wait and hear what other builders have experienced.

jacco vermeulen gave some advise about stability in an earlier post ( 1174). Do you have any comments on that? Read my #1453.
I consider to do this mod, but are unsure of what he means about the location of capacitors, which now sits between drain and source.

Eivind Stillingen
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Old 23rd February 2011, 09:59 AM   #1457
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Post 1174 :

What is most linear power Mosfet around?
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Old 23rd February 2011, 12:09 PM   #1458
Liliya is offline Liliya  Norway
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jacco vermeulen

Thank you. I have read the thread that you referred to.

Hansen wrote: Make sure you are using gate "stopper" resistors as close to the gates as physically possible. As a starting point, these should be somewhere in the range of a few hundred ohms to a few thousand ohms.

Assuming that you are using source followers in the output stage, another point that can help with oscillations is to run a small cap (1000 pF or so) from the drain of each device to the heatsink itself. This idea was taken from the Hitachi application sheet, and will stop oscillations in many cases. It is essentially a local bypass cap on the power supply rails to the closest chassis ground, so it won't hurt the sonics at all.


Gate stoppers: I use 100 Ohm. I know that increasing the value is one way to cure oscillation. Drawback: FETs get slower.

1000 pF from each drain to the heatsinks can easily be done in my Goldmund, but what about the capasitors that already are mounted between drain and source. As far as I have understood this are placed there, not to cure oscillation, but to equalize capasistance to the FETs. Shall they stay, or shall I remove them.
May bee this is a silly question, but Hanson wrote "assuming you are using source followers". When it comes to Golmund, is it???? I presume you know the original schema. I am not familiar with "source follower".

Changing the bias is also mention as a cure in this thread. I went for a bias current of aprox. 140 mA to each device, but have not looked at a scope to see if this (bias) influence on possible oscillation. My FETs are Exicon 10P16/10N16.

Can oscillation in any way be easily registrated whitout a scope?

I have registrated the following: Without any connection to a preamplifier or to speakers, heatsinks get warmer than what happens after hours of listening on relativly high level ( I stayed up to night, listening, difficult to go to bed).
Oscillation????

As you can read from my post to Ostripper, Goldmund clone play(for my ears) music in a very satisfying way.

Eivind Stillingen

Last edited by Liliya; 23rd February 2011 at 12:12 PM.
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Old 23rd February 2011, 12:47 PM   #1459
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Eivind,

only (bypass) caps i've seen in the schematic/boards are 10nF between drain and (star) ground, not the source of the power MOSFETs.

Common ways to compensate for difference in input capacitance are :
- adding caps in the hundred pF range between gate and source
- using different value gate resistors (e.g. 390R and 680R in the Tandberg power amp i linked to)

Only relevant part of Mr Hansen's post to explain what i said in no 1174 :
another point that can help with oscillations is to run a small cap from the DRAIN of each device to the HEATSINK ITSELF. It is essentially a local bypass cap on the power supply rails to the CLOSEST CHASSIS GROUND.
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Old 23rd February 2011, 01:08 PM   #1460
AndrewT is online now AndrewT  Scotland
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The heatsink grounding and decoupling.
The extra cap being referred to must go from the supply to the sink.
An SF (=BJT EF) output device has the Drain attached to supply so an SF requires the drain to be capacitor coupled to the sink.

A common source output device (CFP) has the source connected to the supply, usually via a source resistor.
If the source resistor is in place, which side of it gets capacitively coupled to the sink. The supply side of the source resistor or the source of the output device to the sink?
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