Wednesday, September 3, 2014

Report on a "Goldmund Clone" amplifier from eBay


I recently bought a new power amplifier from a seller on eBay. It was sold as "HiEND power amplifier 195W*2 8ohm ,aluminum,base on Gold*Mund mimesis 29" The amp is rated at 195W into 8ohms, and uses 8 lateral MOSFETs per channel.

The Goldmund design is quite well-known among DIY amp builders. It was said to produce a very good sound quality. However, even the original Goldmund amps (some of them, at least) had a tendency to self-destruct. And the seller seemed to be warning about possible problems - here was his list of warnings:
1) do not only connect one speaker..
2) do not use tube pre-amp.
3) if you want change the signal or the speaker cable,you must turn off the power amp  first
4) when you  playing very very loud sound ,please  touch the heatsink and be sure the heat sink  no too hot so that can cook the egg..

So I went into this with my eyes open. One reason was price - about us$550 including shipping. I asked why no tube preamp, and got no answer. 

I was impressed when it arrived.


http://www.madscientist-audio.com/goldclone1.jpg


All-aluminium case. Large transformer. Speaker protection board. 16 lateral mosfets - these alone cost more than $10 each. It uses 2SK1058 and 2SJ162 MOSFETS. OK, not the original Hitatchi, but lateral mosfets are much better for amplifiers than the cheap switching MOSFETs often used.

And when I plugged it in to play, wow, it sounded great.

Problems..

For a few hours. It actually died while it was idling. Had been on for about 8 hours. One MOSFET blew and took the fuse with it. So I happened to have a few spares handy. 

I figured that the gate-stopper resistors were too small, so I replaced these with larger values - 470R and 820R (different values for the P and N channel MOSFETs, as they have different input capacitance). Gate stoppers are resistors that are meant to stop oscillation, which is a common problem with MOSFET output stages.

So that didn't work.

I spoke to the seller and he admitted that quite a few of the amps he sells have problems, but he claimed that this one had been running fine, in Hong Kong, for a week or more. I noted that HK has 220VAC. My voltage is 240V, so significantly higher. I wondered if this might be the problem.

I decided to try reducing the voltage that the amplifier saw. I used a trick that John Broskie taught me, using a 25 Volt transformer to subtract 25 volts from the 240VAC that the primary sees; this reduced the input voltage to 215VAC, which meant that the rail voltage dropped from 68-0-68 to 58-0-58

This solved the problem. The amp ran just fine now, and still sounded great. I also obtained a circuit diagram of the variant used in this amp. Thinking about what had happened, I figured that the problem was due to the voltage stage getting too warm, which caused it to oscillate at some point, which then caused a MOSFET to die, as the rail voltage was quite close to the MOSFET's max voltage of 140V. 

I tested this theory by making a simple regulator (using 56V zener as shunt) to power the voltage stage. The output and driver stages were still on 68-0-68. This also did not self-destruct, so it seems my theory was correct.


I plan to make a more sophisticated regulator soon, probably some kind of capacitor multiplier.


More Problems...

However, there was another problem with the amp. It uses 4 pairs of parallel MOSFETs per channel. And these are directly connected to the output. Most circuits use a low valued source resistor. This is designed to prevent one pair of mosfets taking all of the load current, using local feedback (degeneration). When Goldmund builds their amps, they match the MOSFETs so that this resistor is not needed. However this amp did not have matched MOSFETs I was sure. I did a high power test and sure enough, I managed to kill the amp again.

So I put in 0.1ohm source resistors. This made it much harder to blow the amp, but I still managed in the end.  Replacing them with 0.2ohm resistors solved the problem.

There you have it - a couple of small fixes and you have a great amp. I was amazed at how good this sounded. Compared to my main amp, the Moskido, a hybrid, non-feedback amp, I find the Clone amp to have more detail, speed, dynamics. But the Moskido sounds more musical. That's the magic of tubes..

And it works now just fine with a tube preamp! 

You Want One?

I'd be happy to explain how to make the changes needed to make this a functioning amplifier. I can also let you have the circuit diagram. I could even do the work for you myself but this would cost in shipping. But still a bargain considering that you'd need to spend a lot more than this to get an equivalent-sounding amp.

Black Discus on the Amplifier

As you can see, I've added a fair number of CanOpeners to the amp. I was trying them on the capacitors. The best place for them is over the heatsinks that cover the voltage amplifier transistors.




6 comments:

  1. Hi Bob, in searching for a method of contact for you I saw the sad news on PIPI and I feel your pain.

    I am currently waiting to receive my clone boards as I did not go the whole hog with a completed amp due to the prohibitive cost of shipping to the UK. Once the courier has added their cost plus the "customs clearance fee", duty paid to taxman and further delivery cost to the door it all gets a bit too much.

    It looks like you have done quite a bit of surgery to your clones and I would be interested in hearing about the components and location that you swapped out on the boards. I am a competent soldering iron user and use books, kits and boards to further my electronics knowledge.

    Can I also ask what the 'CanOpeners" are, is it an anti-vibration aid?

    I look forward to hearing from you in due course.

    Dan

    ReplyDelete
  2. I got this amplifier based on your review.. I"m in the usa with 120v so it seems to have less problems than you had running it on your local power.. it sounds incredible.. I'm using it with a passive preamp with a stepped attenuator and have never heard my speakers sound so good. It was between this and the cheapest emotiva and I wanted something less common so took a chance based on you saying it sounded good. Thanks! Honestly this is the first time I've heard the actual music and not the speakers.. it actually sound like the music is coming from behind and between the speakers now instead of from the boxes. :) took some unboxing pics http://imgur.com/a/lxDeE

    ReplyDelete
    Replies
    1. ..and you know there is quite a lot of performance still to be got from this amplifier. I should do a blog update with my latest mods - including a 1600W power supply. It's really high end now.

      Delete
    2. I would add the gate-stopper resistors though - even if you don't have problem with power supply. Otherwise you might kill it if you play too loud. With gate stoppers it will not even blow up when running at 400WRMS

      Delete
  3. hi Bob!

    I have a goldmund 29 clone and i absolutely love it. I need to replace the speaker drivers (lowther ex4) and would like to go with the 15 ohms drivers this time. My question is.. is a 15 ohm load ok without any chance blowing the amp? I really dont have the skills to make any large mods of the amp...

    ReplyDelete