Mullard Audio Circuit

[Radio Wrangler]

These are all a lot beefier than that Mullard not-quite-10-Watter. and liable to be a lot more expensive to build.

The Mullard one is definitely not much good, and I think DaveW would be wasting his time if he decided to build one. He might as well spend his effort on something likely to have a useful outcome.

Does anyone know of a good straight-forward design in the 10W power range?

David

[Craig Sawyers]

There are a bunch of early Cambridge Audio designs here http://www.angelfire.com/sd/paulkemble/sound8i.html

The original P40 (20W per channel) is worth a look. It was thought to be unreliable, but that was down to the way the power transistors were mounted just on the thin chassis, so lousy heatsinking. Otherwise it was well thought off in sound quality, and rather radical for its day.

I have one - must haul it out and measure it.

Craig

[mhennessy]

To be honest, the original concept is not all bad.

Philips used an even simpler version of that that circuit in their N2400 stereo cassette deck - we had one at home, and used it as a mini hi-fi amplifier, feeding a radio into it. It sounded nicer than the cheap Japanese music centres that people were throwing away in my direction during the '80s. It came complete with the schematic folded up inside, and I remember cloning it really quite successfully. It was rated at 4 or 5 watts per channel, so some marketeers might describe that as 10 watts peak power, and the difference is only 3dB anyway - it certainly went loud enough for me at the time (I was only 10 or 11 IIRC). Obviously it's not a patch on the Self circuits, but for someone wanting to learn about how amplifiers work, it's not a bad starting point - it would work with modern silicon transistors with minimal modifications. "D" is 25V at idle. No idea what the distortion was like, but this was a cassette deck, so you can be sure the distortion level was appropriate for that context. In the commercial world, "balanced" design is better than "best" design.

However, if you're not planning a "journey", and instead would like to build something that works and will be your "forever" amplifier, then the options are simple:

1. For around 50 watts or less, a pair of LM3886 chips.
2. For more than that, a Self Blameless. If you can find all the parts.

If you haven't already, get a copy of Self's book. Whether you build one of his or something else - or indeed, nothing at all - it's essential reading.

[G6Tanuki]

Personally, I like "Simple" - in the past I've used a kind-of scaled-up version of the classic 3-transistor PNP/NPN complementary-symmetry amp you'd find in millions of 1970s/1980s transistor-radios/car-radios - but using 115-Watt-rated 2955/3055-type TO3 silicon transistors, with 'serious' heatsinks to keep things cool.

A couple of forward-biased stud-mount diodes on the heatsink were OK for thermally-compensating the forward-bias on the output transistors.

[Craig Sawyers]

Originally Posted by mhennessy

2. For more than that, a Self Blameless. If you can find all the parts.

If you haven't already, get a copy of Self's book. Whether you build one of his or something else - or indeed, nothing at all - it's essential reading.

The only tricky thing with the Blameless are the power devices, the original ones and their replacements being both obsoleted. However the easily available MJL1302A/MJL3281A are Self's current recommendations. And are listed in his book. About £3 each from Farnell etc. A pair of these are good for well over 100W at ridiculously low distortion in a Blamelss. With a chunky finned heatsink of course. You can even parallel those transistors up to feed current hungry low impedance speakers.

If you don't have a current generation Audio Precision test set (I don't, but Self does) you stand no chance of being able to measure the 6ppm distortion of a Blameless.

The power amp and small signal audio books by Self are both excellent. I've just bought his active crossover one too.

Craig

[Craig Sawyers]

Back in the day the Texan, published in Practical Wireless in the early 70's was a massively successful self-build. It used a mix of 741 and 748 op-amps (They were 60p then - so £6 each now!). The power amp was really simple, being basically an op amp, a vbe multiplier, driver transistors and a couple of power transistors, with the feedback fed back to the opamp. Split rails, so no output capacitor, and 20W at relatively low distortion, and stable.

Mine is still in the attic. My dad used it for many years after I moved on to ever more ridiculous audio foibles.

Info about the Texan is here http://www.angelfire.com/sd/paulkemble/sound8h.html

Craig

[G6Tanuki]

The PW Texan was a practical and worthy 'first-cut' implementation of an amplifier based around a Texas Instruments design-brief.

I know people who built them - and soon came to understand that the designers were clearly inhabitants of a strange and unique world in which there were no random wideband-noise clicks from thermostats, or RF-interference from passing taxis/CBers/police/ambulance/water-company/coastguard/aircraft radios.

[Radio Wrangler]

Notice that the output stage of the Texan isn't just a unity gain emitter follower pair, it is a pair of complementary feedback pairs with a voltage gain of about 12.

An odd circuit, but it seemed to work well enough.

David

[mhennessy]

Originally Posted by Craig Sawyers

The only tricky thing with the Blameless are the power devices, the original ones and their replacements being both obsoleted.

No, as I mentioned in post #19, I'm talking about the various high-voltage, small-signal transistors, such as the MPSA56, which has been last-buy/obsolete for some time.

Though I note that Central Semiconductor appear to be making them now - no idea how these compare to the originals, but that's good to see. This must be a pretty recent thing, as I was searching for stocks of the MPSA56 just a few months back, and no-one had anything apart of NOS. Whether I'd specify them for a new design just yet remains to be seen...

https://my.centralsemi.com/product/p...hp?part=MPSA56

[Argus25]

Originally Posted by mhennessy

Usually for this sort of output stage, the Vbe multiplier has to be thermally bonded to the drivers, not the output devices.


But there's no getting away from the fact that you have to put more effort than usual to ensure they are stable.

I should have mentioned that the drivers and outputs are thermally bonded together and the Vbe multiplier bonded close to all.

And yes, it does require more effort to ensure they are stable. But when it is, it works fine.

[Argus25]

Originally Posted by Radio Wrangler

Notice that the output stage of the Texan isn't just a unity gain emitter follower pair, it is a pair of complementary feedback pairs with a voltage gain of about 12.


David

Yes, just like the ETI-480, but with a higher gain in the output stage.

[Craig Sawyers]

Originally Posted by mhennessy

[QUOTE}
No, as I mentioned in post #19, I'm talking about the various high-voltage, small-signal transistors, such as the MPSA56, which has been last-buy/obsolete for some time.

I stocked up on the higher power (1W) versions, the MPSW56 and MPSW06 a year of more ago. These are in the tall versions of the TO92 case. So I have enough stock to see me out. On-Semi have obsoleted them too.

They do still live on though - but now only in SOT surface mount versions.

You are right though, there are two manufacturers of the TO92 packaged versions Micro Commercial Components and Central Semiconductor.

Craig

Craig

[Argus25]

Originally Posted by G6Tanuki

I know people who built them - and soon came to understand that the designers were clearly inhabitants of a strange and unique world in which there were no random wideband-noise clicks from thermostats, or RF-interference from passing taxis/CBers/police/ambulance/water-company/coastguard/aircraft radios.

Exactly this happened though when amplifiers transitioned from germanium to silicon devices for audio work. The Germaniums were intrinsically RF proof and totally deaf to RF. On the other hand the silicons had transition frequencies in the many tens to hundreds of megacycle region. Also with high level interference non-linearity would do a great job of rectifying amplitude modulation.

So all of a sudden all sort of interesting radio communications and clicks & pops appeared at the outputs of the silicon transistor amps.

I remember a large setup of amps for a rock concert in a (freshly made with Silicon devices) in a public park, and you could hear practically every transmission in the area, it was hilarious.

Then the designers learnt that to change a MJ2955 into an AD149, from the frequency response perspective, you put a 0.1uF capacitor from its base to collector ! And all the next generation amps had small value ceramic B-C capacitors scattered through them on the transistors in an attempt to muffle the RF response.

This is one reason why an internally compensated vintage OP amp like a 741 rarely misbehaves and makes a great hobby part for a novice, it cannot, it can barely even amplify the upper audio spectrum successfully and its slew rate looks like the side of Mt Fuji.

[Max Ripple]

I too still have my Mullard audio book's.These are well thumbed of course.I built various of the amplifiers,but in fairness to Mullard,I think that the circuits were design suggestions utilising Mullard devices,and probably not fully evaluated by them?.I remember all sorts of stability problems encountered,both signal and temperature.It was all part of the journey of course,overcoming these problems,and I learnt a great deal in the process.
I still have (somewhere!)a little stereo record player that I built for my then teenage Sister(now a Gran!!!).It uses a Mullard amplifier and a Garrard 2025 autochanger(inc Acos GP104).I remember listening to Miles Davis etc via this system,and being pleased with the sound quality.
Happy day's!
Nick

['LIVEWIRE?']

The only version of the Mullard transformerless amplifier I ever made was a low-power one using all germanium devices (AC127,128 and AC128/176, IIRC)back in about 1972 or 73 to use with a multi-band MW/SW receiver from a PW design, instead of the transformer-coupled audio amp. in the published circuit. The amp. I built had a max o/p of about 0.5W into 8ohms, which was perfectly adequate for SWL-ing, as was it;s freq. response. I still have it somewhere, though the germanium transistors may be suffering from the 'tin whisker' problem by now.

[kalee20]

Originally Posted by Radio Wrangler

Notice that the output stage of the Texan isn't just a unity gain emitter follower pair, it is a pair of complementary feedback pairs with a voltage gain of about 12.

An odd circuit, but it seemed to work well enough.

It also has the benefit of nearly rail-rail output swing, without the voltage drops associated with emitter-followers.

[mhennessy]

Originally Posted by kalee20

It also has the benefit of nearly rail-rail output swing, without the voltage drops associated with emitter-followers.

Which, although potentially helpful to the marketing department, is, as I alluded to in post #19, of no practical benefit whatsoever.

It might be different if it was running from 3V battery supply, perhaps. But not in a mains powered hi-fi amplifier.

[kalee20]

Originally Posted by mhennessy

Which, although potentially helpful to the marketing department, is, as I alluded to in post #19, of no practical benefit whatsoever.

You did indeed Mark! And it can also be a bit more fun to stabilise again as you pointed out (been there done that - making a low-dropout voltage regulator rather than an AF amplifier).

However, in the Texan, the output stage is driven from a 741 operating with -15 / +15V rails, so the output stage has to have some voltage gain else it would be limited by the +/- 13V swing of this, irrespective of the 25V rails to the output.

[Argus25]

I've attached EA's classic circuit for a 10W stereo amp from 1969 with the AD161/162 outputs. Everybody was reading the Mullard "how to" book back then. These sorts of amps were quite popular with hobbyists in Australasia and compared to amplifiers that came before them, they really did look like "state of the art" at a good price.

We could judge them harshly by today's standards, but in the context of the time they were made, they were pretty revolutionary and you could listen to your Yardbirds or Rolling Stones LP's without any difficulty. Then if you really wanted to go the whole way you could build a color organ to go with it and the amplifier could send you to a transcendental plane (outside working hours that is).

[ColinTheAmpMan1]

Originally Posted by G6Tanuki

The PW Texan was a practical and worthy 'first-cut' implementation of an amplifier based around a Texas Instruments design-brief.

I know people who built them - and soon came to understand that the designers were clearly inhabitants of a strange and unique world in which there were no random wideband-noise clicks from thermostats, or RF-interference from passing taxis/CBers/police/ambulance/water-company/coastguard/aircraft radios.

I, too, built a PW Texan. IIRC, Henry's Radio (on Edgware Road?) sold a kit. It was a good amplifier for several years in my possession and I never recall any noise or interference - maybe I was lucky? I sold it on to a friend after I got a NAD 3020, which is soldiering on still.

Colin.