VM100 assembled version of K8060


Hello to all, in case someone reads :wink:

So we purchased 4 VM100, the factory pre-assembled version of K8060.
Including original Velleman toroidal transformers.

Subsequent assembly was carried out in a professional workshop by experienced technicians with forced cooling fan and fully grounded aluminium enclosure.

The first still behaves like a charm and is used in the frame of scientific research as instrumentation amplifier.

Second one burst in flames shortly after power-up, meaning maybe 20 seconds. Aftermath showed a probably poor thermal connection between one transistor and the heatsink. No bias was adjusted since the operator manual specifies that the module is adjusted (and therefore implicitly) powered-up at the factory. So a warranty claim was filed to the distributor.

Third and fourth of absolutely identically assembled devices are unusable for the following reason:
-The output is HIGHLY DISTORTED After the input (fed by a Tektronix function generator) reaches about 200mV Peak-To Peak on a pure sine wave @ 150 to 300 Hz.

We took oscillograms. All is fine below approx. 200mVpp at the inpout then, as we slowly approach the critical threshold, negative sharp spikes progressively appear on the first half period, shortly after the sinus wave starts to decrease from it’s maximal output value of about 6 Vpeak.

At power up, with the input left open or shorted with a 50 Ohms BNC cap, the output will go haywire for a moment, violently swinging at higher frequencies between V- and V+ for a few seconds. Then it’ll stabilize within reasonable limits to ground.

On occasion the primary fuse, 1A slow burn will blow at powerup.

We measured the primary current using a Textronix hall effect current sensing clamp and found a highly non-sinusoidal behavior with very sharp spikes, rising up to 600mApp on a idling device.

Transformers behave as if badly saturating even at idle at this point of the investigation.

Same picture on the secondaries. Sharp spikes of meaningless peak to peak current values well synchronized with the commutation points of the rectifier diodes. Remember, there’s no input at all connected for this test, device is IDLE.

We checked of course the secondaries phase, and symmetry, all matches within the usual couple of percent of amplitude difference between the secondaries.

Voltage exhibits a better, close enough to sine wave signal.

Further we placed a series of chokes on the primary and could smoothen a bit the current spikes enabling us to operate one device till about 240mVpp sine wave on it’s input @ 150Hz.

Yet on powerup, without signal or with a 50 Ohm BNC cap, with or without any, including pure resistive 5 Ohms load, the output will go haywire for a second or so, blowing the 1 A slow fuse on the primary in most cases.

What re we facing here ? A vicious transformer malfunction, airgap in the magnetic circuit ?

The current oscillograms trend to show that the magnetic circuit behaves as being extremely saturated, unable to cope with even it’s own idle current.

Now, this is VERY BAD since equipment is delayed by mire than one week and we need it to perform ASAP.

Can someone help, is there a support for these products ? I personally phoned in Belgium. No service other than this forum, officially said, evidence available on request.

Thanks, let’s see where this will lead ? We of course won’t alter the remaining VM100 modules unless specifically instructed to do so. We both them as ready to use modules under warranty.

Best regards, thanks for the attention,



I have two of these amps running flawlessly in my guitar amp fed by a used bandtransformer with double outputs.
Last week I assembled two new kits and checked them with a symmetrical lab supply to check for malfunctions…all OK.
Built in an old amplifier cabinet with +/- 40 Volts everything looked fine until turning up volume of test sine on 7,5 Ohm resistive load. At about 20 V. pp the top half sine showed signs of oscillation around the top. Closer inspection showed saw edges trailing to about zero Volts at high frequency. Looks similar to complaint above… any suggestions?
Power transistors are somewhat different built-wise.

EDIT: Took an amp from the cabinet and connected it to another supply with same results. Top half of the sine oscillates starting at the top and broadening to both sides with increasing input. When I lower the supply voltage the oscillating part becomes smaller towards the top of the sine before disappearing altogether.


In addition I can say that the oscillation on the top half of the sinewave follows the shape of the sine and the 0 Volt line.
When turning down the positive supply voltage the oscillation splits up in two halves ( like a cat eye tube) and disappears.
When turning down the negative supply voltage the oscillating area becomes smaller and disappears.
Both before clipping starts on positive / negative half.
It seems the PNP power transistor takes no part in the oscillations.
Any ideas?


It sounds like the voltage maybe to high.

These are the specs.

200W music power @ 4 ohm load
100Wrms power @ 4 ohm load
70Wrms power @ 8 ohm load
distortion: 0.02% @ 1KHz/10W
damping factor: >800
frequency response: 3Hz to 200KHz (-3dB)
sensitivity: 0.6Vrms
signal to noise ratio: 115dB

[color=#BF0000] power supply: 2 x 25-30Vac / 120VA[/color]
PCB dimensions: 4.2 x 2.4" (approx.)
recommended heatsink: HSVM100 (pre-drilled) or S44/75 (not drilled)
recommended power transformer:
230VAC input, 2 x 30VAC output: 12030
115 / 230VAC input, 2 x 25VAC output: TR8040 (output power reduced by ±15%)


Thanks for your reply. In another thread I read the V max is +/- 42 V.
I’m below that with my symmetrical lab supply in test.
Below is a scope picture of what a 10 kHz. sine wave looks like. At 10 Hz it’s the same with more spikes. The signal voltage level seems to be the culprit that triggers the oscillation.


I would like to revive this thread and ask if anyone ever had this problem. The two earlier built still work without a problem and these two give the above and below

scope picture on multiple supplies and voltages…


Please indicate if multiple amps are used?
Always test 1 amp with 1 power supply
Multiple amps with multiple power supplies may result in ground loops, this will cause oscillation like picture.


The problem occurs with one amp in an old amp casing but also on the workbench with a symmetrical lab supply. Yesterday I found that the supplied idling current pot is 1k instead of 500 R and I’ll check if changing it to the original 500 R will change anything. I was planning to eventually use two as I’m used making ground plans that work. If i experience pproblems then I will not trouble you as I was warned beforehand.
Thanks for your reply :slight_smile: