Anyone good at fault finding with amplifiers here?

[Neil WyattModerator @neilwyatt]

Posted by Joseph Noci 1 on 19/08/2019 07:51:45:

By the way, Neil, Your amp seems to have been a Maplin Kit in the late 1980's – the circuit and some descriptive comments are in Ian Hickman's book 'Analog Circuitry Explained' – published 1990.

Joe

I've got Michael Tooleys' Maplin Electronic Circuits Handbook, which has a section on that kit. I came within a whisker of making it a few times!

The basic idea is the same, although the biaising is different and there are detail component differences everywhere. There's also a design for a rather less powerful (about 40W RMS) version using a pair of power darlingtons earlier in the book and between the two descriptions you can get a good idea how this configuration works. Essentially, they (and the Trace Elliot design) are high-power op amps made with discrete components! Relative simplicity, good zero crossing performance and accurate feedback makes them very low THD and excellent for demanding applications.

I've made two of the bipolar version, somewhere I have a design for the board, one to replace the power amp in a Carlsboro guitar amplifier and one for a little 'uAmp Micro Bass Combo' I made from scratch.

Back to the TE design, mine is the original 150W version with two fets and a fan. The ones I have the schematics for include a 150W version with four FETS and no fan and a 250W one with four FETs and a fan.

I could uprate to 250W simply by getting hold of an uprated transformer and two FETS and two resistors…

Neil

[AdrianRParticipant @adrianr18614]

The Maplin amp from the 80's, sounds like the ones I built. Fantastic little amps. With the preamps I designed I had so little hum. Even at full volume and no input you had to put your ear against the speaker to hear it.

Now a 4Ohm 100W resistor for a dummy load has turned up, my next job is to fix a 100W Traynor valve amp. Will be fun to play with valves again.

Adrian

[Neil WyattModerator @neilwyatt]

The +/- 69V DC power rails seem awfully generous for a 150W amp.

I just had a look, and the amp is powered by a massive transfroer from the 50-0-50VAC terminals, not the 39-039V ones that would suit a 150W amp.

I'm now wondering if the reason it wasn't fitted with sockets for an external cab was because it only has two instead fo 4 mosfets.

Strikes me it would be sensible to fit a second pair seeing as the PCB has the locations for them if I'm running it at 4R.

Neil

[Neil WyattModerator @neilwyatt]

I found a thread, in Dutch, about someone with the same problem, except their DC offset was only 127mV.

Much discussion and debate until in the last post they replaced TR201/2 with a matched pair and the offset came down to 5mV, then replaced TR203/4 as well and it came down to 3mV.

I have high hopes…

There should be a translated version of the thread here:

Thread on Dutch forum

Neil

[Joseph Noci 1Participant @josephnoci1]

I see that Dutch fellow did sort of as I asked you to do – gently heat the long tailed pair individually with a soldering iron – except he cooled them with a frozen pancake ….Ah well, whatever flaps your jack…

I am sure your problem is simply gain mismatches in the long tailed pair(s).

The math shows it, so why not. Just fiddled a bit with LTP's in LTSpice ( Linear Technologies, now Analog Devices) free version of Spice – just cause its easy, lazy and quick.. Playing with the gains, with large loop gains, give BIG offsets for small hfe deltas.

When will you get your tranny's?

What intrigues me is ( assuming the amp's original offset was 'good' ) what causes it to change? Unless the transistor is abused, over-voltaged, overheated, etc, hfe should not really change much in years, and in a matched pair, the changes should be similar in both. From the circuit values I've seen , typical of your amp, current levels and Vce are well under control, so no abuse there. Interesting. I also remember, many years ago when making accessory widgets for our 'band' , I made a white noise generator, feeding voltage controlled filters to generate ocean sounds..but, noise eluded the device, Thinking the device stuffed, I used the transistor tester to verify – The transistor was ok, with hfe around 370. So I took a cigarette lighter and heated the BC107 till it glowed .. after cooling, I tested it again, still ok..and hfe had changed to 368….!!..within measurement error…

But,boy, did it now make nice ocean sounds….

Joe

[SillyOldDufferModerator @sillyoldduffer]

Posted by Joseph Noci 1 on 20/08/2019 14:54:29:

…

What intrigues me is ( assuming the amp's original offset was 'good' ) what causes it to change? Unless the transistor is abused, over-voltaged, overheated, etc, hfe should not really change much in years, and in a matched pair, the changes should be similar in both.

…

But,boy, did it now make nice ocean sounds….

Joe

Maybe the can's seal gradually failed over the years and the innards are exposed to dirt and moisture. Perhaps Neil's transistor has gone rusty.

Ocean sounds invariably produced by my amplifier efforts too. Motorboating…

Dave

[Neil WyattModerator @neilwyatt]

You can tell it was a Dutch forum. A Frenchman woudl have used a frozen crepe, and a Belgian would have pointed a hot waffle at it.

Hopefully replacements arrive tomorrow.

It seems other folks have added the 'missing' power FETs as well.

I remember removing the cans from AC127s to make phototransistors – choose the ones where the blue heatsink compound doesn't cover all the chip.

Neil

[Neil WyattModerator @neilwyatt]

Went back through some old threads where I had been going on about how the amp was putting out a lot more power than its rated 150W (the legendary 'Trace Elliot Watts&#39.

Found a post by an ex-TE engineer I should have read properly.

Some 150W two-fet boards were put out with the PSUs for 200W/300W into 4ohm amps.

With a 50-0-50 transformer and PSU capacitors rated 4700uf/100V instead of 63V mine is obviously one of these 'snowflake' amps.

Presumably this is why the external speaker sockets weren't fitted but a fan was.

So add my two extra FETs and I have a rare-as-hen's teeth TE 300W combo.

Neil

[AdrianRParticipant @adrianr18614]

Back in the day when making differential amps it was common to glue the two transistors together so the temperatures were the same. I doubt your board will allow that, but you could glue a small bit of aluminium or copper between the pairs.

If you still have an offset you could inject a DC offset to T202. As you dont have a separate regulated supply, you will need to decouple it with a capacitor.

Adrian

[Neil WyattModerator @neilwyatt]

Posted by AdrianR on 20/08/2019 18:37:36:

Back in the day when making differential amps it was common to glue the two transistors together so the temperatures were the same. I doubt your board will allow that, but you could glue a small bit of aluminium or copper between the pairs.

If you still have an offset you could inject a DC offset to T202. As you dont have a separate regulated supply, you will need to decouple it with a capacitor.

Adrian

I'll be happy to get it to a fraction of a volt, it's not hifi, I just want to make sure i can sue all the headroom and keep it running a bit cooler

Neil

[Neil WyattModerator @neilwyatt]

The replacement transistors have come.

The gain spec for the BF423 is >50, the two fitted are 80 and 64, but is suspect at least one is partially fried.

Out of interest the new ones are (first measurements, repeats come out +/-1):

129 – 2

134 – 2

135 – 2

136 – 1

138 – 1

139 – 1

151 – 1

The 151 is interesting, otherwise they are all pretty close for a random selection.

And… the output offset voltage is now 0.22V, a bit higher than I hoped, but now no visible displacement of the speaker cones. I will replace the pair of BF422s and adjust the bias current when I fit the two extra FETS, but I need to wait for the mounting kits to arrive.

Neil

[Joseph Noci 1Participant @josephnoci1]

That's a good result Neil. So, You found the mismatch and there was not much hfe from the rest of us…

Joe

 

– if you want to get the offset down, and mind not being a scoundrel, warm each transistor with a soldering iron ( did I say this before??) – one of them will make the offset reduce – then, the scoundrel part, heat that one hot and let it cool and see where the offset ends up – repeat with greater heat each time till happy..

– You mentioned the phototransistor trick – I did the same but used OC70's – nicked out of modern transistor/valve hybrid car radios, themselves nicked from the local scrapyard where cars went to die. Remember those mechanical vibrators for the valve HT? And OC35's in a multivib stage that replaced the vibrators? Got a lot of electronic bits from that scrapyard's car radios in my kid days..

Edited By Joseph Noci 1 on 21/08/2019 15:16:34

[Neil WyattModerator @neilwyatt]

I heard of those HT units, but don't recall seeing one.

I bought a (new) dual valve preamp a few months ago, just because it was ridiculously cheap, runs off 12V stepped up with a more modern charge pump on board. Ideal for sticking a valve overdrive emulatoir into a transistor guitar amp, methinks. Only has a gain of about 2, apparently (no documentation with it), but easy enough to stick a high-gain transistor stage in front.

"so many projects, so little time"…

[Enough!Participant @enough]

Posted by Neil Wyatt on 21/08/2019 16:02:08:

"so many projects, so little time"…

…. wait till you get to the average age around here

[ "Age" might make an interesting (anonymous) poll. ]

[Joseph Noci 1Participant @josephnoci1]

Posted by Neil Wyatt on 20/08/2019 18:11:21:

Went back through some old threads where I had been going on about how the amp was putting out a lot more power than its rated 150W (the legendary 'Trace Elliot Watts'.

Found a post by an ex-TE engineer I should have read properly.

Some 150W two-fet boards were put out with the PSUs for 200W/300W into 4ohm amps.

With a 50-0-50 transformer and PSU capacitors rated 4700uf/100V instead of 63V mine is obviously one of these 'snowflake' amps.

Presumably this is why the external speaker sockets weren't fitted but a fan was.

So add my two extra FETs and I have a rare-as-hen's teeth TE 300W combo.

Neil

I presume your posted schematic is complete in its output stage, ie, not a 'representation'? I would expect good design would have placed current sharing resistors in each FET output leg when more FETs are added in parallel.

Not sure it is a good idea omitting these resistors..(0.2ohm to 0.33ohm 5watt for your amp)

Joe

[AdrianRParticipant @adrianr18614]

I presume your posted schematic is complete in its output stage, ie, not a 'representation'? I would expect good design would have placed current sharing resistors in each FET output leg when more FETs are added in parallel.

Not sure it is a good idea omitting these resistors..(0.2ohm to 0.33ohm 5watt for your amp)

Joe

Joe,

It is vital to have the resistors for BJTs, as they have a positive temperature coefficient which means the current increases with temperature. MOSFETs do not need the resistors to compensate for temperature effects. MOSFETs have a negative temperature coefficient which means the current decreases as they get warm, this means they self balance.

Adrian

[Joseph Noci 1Participant @josephnoci1]

Posted by AdrianR on 22/08/2019 08:06:20:

It is vital to have the resistors for BJTs, as they have a positive temperature coefficient which means the current increases with temperature. MOSFETs do not need the resistors to compensate for temperature effects. MOSFETs have a negative temperature coefficient which means the current decreases as they get warm, this means they self balance.

Adrian

Hi Adrian,

Unfortunately that is a common misconception, and under many conditions, quite untrue…

Here is a small excerpt from a very good paper in the supplied link:

With the push for faster switching, lower on resistance power MOSFETs, came an unintended consequence similar to, but not seen since the prime of the bipolar transistor, which was the secondary voltage breakdown effect. While MOSFETs are in the charge-carrier dominated region (low Vgs) the MOSFET allows more current to flow as the temperature increases causing a thermal runaway. It was discovered that the SOA curves given by the manufacturers were lacking in giving the region of thermal instability. A review of papers from the automotive industry is described, and recommendations to add the area of thermal instability are included. The four factors that are important in determining the thermal instability are:.

etc, etc….

Link – Mosfet Thermal Runaway

Even the venerable Douglas Self points out the existence of the problem..

FET output devices are not the gift to ears that AudioPhiles seem to believe they are…

They exhibit far greater distortion compared to their bipolar brethren ( anywhere between 10 and 20 times more..), and require much higher bias/standing current to get even close – for a 50 watt single pair output stage it is not uncommon to see bias currents in excess of 150mA to 180mA. Some impressively complex 'correction' circuits have been designed by the Passionate, to try correct this problem, making the resultant amplifier excessively complex. ( If interested search for Error Correction Output Power Stage is based on Bob Cordell’s design, which in turn is based work by prof. Hawksword)

However, my comment on the lack of Source resistors what not aimed at the issue of potential thermal runaway at all..The issue is:

The Gate threshhold voltage variation is embarrassingly wide, and varies even between so called 'matched pairs' , (Also tends to vary with temperature as well, making one chase the 'ideal' operating point up and down the street.) So, it is essential to match ALL fets in the output stage as closely as possible in gate threshold voltage AND transconductance to ensure minimum distortion. But a 'good' match is still not good enough, and the Source resistors helps to cope with that last bit of imbalance, therefore very necessary..

In most designs by credible designers and manufacturers of Audio Amplifiers, if they do a FET output version, you will find Source resistors.

 

In my book, the only advantage of FET's in the output, is that they more often than not fail open, protecting the speakers a little in a circuit without protection elements..

Joe

 

Edited By Joseph Noci 1 on 22/08/2019 10:41:25

[AdrianRParticipant @adrianr18614]

Joe,

Thanks for that, an interesting read, my excuse is all this came out after I learnt about MOSFETs.

It does infer that the early MOSFETs were stable, but the newer high switching speed ones developed this problem. Neils amp is probably stable as it is quite old, but it could probably benefit from having its heat compounds replaced on the output transistors.

Adrian

[Joseph Noci 1Participant @josephnoci1]

Hi Adrian,

I have no doubt his Amp will be thermally stable – I doubt he will use it at full volume with staccato material for hours on end anyway.. The issue is really about distortion, matched pairs ( or matched QUAD in this case) and trying to balance out small transconductance deltas.

I also wonder what the differences between a current date manufacture set of FET's will be, staked up against the old pair in the amp..and how he will match them! Half decent matching should be done with the FET on a heatsink, at a reasonable temp, maybe 35-40degC, and around 1amp or so as the target current, and then selecting FETs with 'identical' gate voltages that give that current..

I suspect Neil will just ignore all this, and fit the FET's! Would be nice if he could measure the distortion before and after..

Joe

[Neil WyattModerator @neilwyatt]

Posted by Joseph Noci 1 on 22/08/2019 11:15:38:

I suspect Neil will just ignore all this, and fit the FET's! Would be nice if he could measure the distortion before and after..

Joe

Well he will be renewing the thermal compound…

But an extra day or two to wait as I ordered TO3P mounting kits.

No source resistors – the Laney proBass (similar design) uses FOUR pairs of fets in parallel with no balancing.

My FETs are Exicon ones designed for high power audio, not switching, and won't be biased at the low gate voltages which seem to cause problems.

| I have no doubt his Amp will be thermally stable – I doubt he will use it at full volume
| with staccato material for hours on end anyway..

Err.. I thought that was the point

Neil

[Neil WyattModerator @neilwyatt]

ECF10N20, ECF10P20, ECF20N20 & ECF20P20

the ideal choice for audio amplifier design

EXICON Mosfets have been designed specifically for high power linear use.

They offer high voltage capability, high slew rate and low distortion, making them the ideal choice for audio amplifier design.

Freedom from secondary breakdown and thermal runaway make them extremely reliable and remove the need for protection circuitry.

These advantages together with wide bandwidth, low drive requirements, and ease of paralleling make it possible for the simple construction of robust amplifiers with excellent sonic characteristics.

• Specifically designed for audio amplifier applications
• High thermal conductivity
• Excellent frequency characteristics
• Integral protection diode
• Free from current concentration, resulting in a high resistance to electrical destruction
• Temperature characteristics which inherently protect against short circuit fault conditions and thermal runaway

http://www.exicon.info/products.php

[Anonymous]

MOSFET secondary breakdown is known as the Spirito effect and occurs primarily at high Vds and low Id. Most modern MOSFET dies are not a single transistor, but contain hundreds or thousands of small MOSFETs in parallel. The effect is dependent upon the actual MOSFETlas it results from changes of threshold with temperature, and how quickly the cells can get rid of heat, which depends upon the transient thermal impedance.

Andrew

[Neil WyattModerator @neilwyatt]

P.S. those Exicon FETS appear to be the recomennded equivalent for the original 2SK135 and 2SJ50.

Helpfully the PCB has 135 and 50 written on it in felt pen which will make putting the right one in the right place easier but I AM going to trace the connections just in case… with electronics, paranoia is good.

Neil

[Joseph Noci 1Participant @josephnoci1]

Neil's quote re the suitability of those  FETs..

Freedom from secondary breakdown and thermal runaway make them extremely reliable and remove the need for protection circuitry.

EVERY FET spec from every manufacturer, switching FET or not, makes the same claim, and yet…

What is the gate/source voltage in your amp Neil? What is the standing Drain current? Not easy to measure I suppose.

Joe

Edited By Joseph Noci 1 on 22/08/2019 17:34:41

[duncan webster 1Participant @duncanwebster1]

My radio ham friend reckons that when you run out of heat sink compound you can use copperslip. Anyone got any thoughts?

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