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Old 18th Oct 2019, 7:07 am   #21
thermionic
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Default Re: Single-ended Transistor amps.

I totally agree with Terry here.

If you want to build a 10 Watt amplifier using 4, bog standard transistors, and a small compliment of other junk box parts, then you must try this.

I built one many years ago, and I still listen to it now. Iíve listened to many other HiFi amps over the years, but the Linsley Hood holds its own with ease!

Iím looking forward to seeing what you can cobble together!


Cheers. SimonT.
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Old 18th Oct 2019, 6:36 pm   #22
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Default Re: Single-ended Transistor amps.

I'm looking at the Linsley Hood circuit right now, as a possible path. It has some significant attractions. But remember that my [historically explosion-damaged] hearing means I'm not after "Hi-Fi".

As Argus25 mentions, single-ended transformer/choke-load amps can potentially 'spike' and deliver excessive (possibly fatal) instantaeous Vce to the output-device [In times-past when I built 3.5MHz transmitters using an AUY10 as the PA-transistor I lost a couple of these when the antenna became disconnected and the 'tank' circuit turned into a 100%-reflected-energy resonant circuit].

Given that I'm likely to be driving this beastie to the point where on peaks the collector-current gets cut-off, my mindset is to include 'clamp' diodes across the output to minimise the risk of this (remembering how many 1980s/1990s VMOS power-transistors had built-in Zeners to protect against excessive back-EMF).

I've just dug out an old PCB (recovered from a commercial-duty A3 laser-printer a while back) which has a couple of nice little heatsinks. Seeing these, I'm thinking perhaps I should aim for 10 Watts of audio-output?
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Old 18th Oct 2019, 6:52 pm   #23
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Default Re: Single-ended Transistor amps.

As a teen in the '60s I built a single-ended amp using the circuit (car radio output stage?) from the Mullard book of transistor circuits. I wound the output transformer by hand - it probably used the materials from a frame output transformer. The winding details must have been given in the Mullard book. I really don't know why I didn't build a second for stereo use and I can't even remember what happened to the first.

You've got me tempted to re-create this.
Graham
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Old 18th Oct 2019, 9:31 pm   #24
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Default Re: Single-ended Transistor amps.

I built a kit radio for my A35 van using red spot, white spot and a substitute OC16 transistor.
The radio was stolen by breaking the door lock. I was not a happy chappy.
I still have the Repanco details somewhere if anyone is interested.
The radio had poor transistor biassing and would only work well at room temperature. I spent a lot of time on transistor theory and ordered parts to solve the problem but the radio went just before I was going to attack it.
The circuit was made by soldering to brass eyelets fixed in Paxolin.
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Old 19th Oct 2019, 12:31 am   #25
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by FERNSEH View Post
Example of a class A single transistor output stage.

Pye stereo radiograms models 1206 and 1208.

Output transistor is AD140, replacement is AD149.

DFWB.
Thanks for that, i was half remembering those grams, the pa heatsink was a whopping square sheet of aluminium, pretty decent sound as i recall.
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Old 19th Oct 2019, 1:22 am   #26
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Default Re: Single-ended Transistor amps.

I have attached a copy of Mullard's original document supporting the OC16. Not much of a power transistor by today's standards, but then it was revolutionary and I still like it.

This is the circuit I used in the radio I posted a link to in post 7.

There is a lot than can be learnt from this circuit that can be generalized to higher power versions.

In this case the quiescent emitter current is 400mA. This results in a 0.88V drop at the emitter, which is only about 6% of the supply rail...so its not really needed to have an elaborate bias arrangement to eliminate this. The AC gain is reduced a little by the NFB resistor Rf, and the stage produces a very faithful sine wave <5% distortion, which I cannot hear. You can tame the higher range frequency response with a capacitor in parallel with Rf, or make a tone control there.

As stated it requires a heatsink of 5.5 degC/W or less. ( in my radio I increased Re to 3.3R increasing the emitter drop to 1.38V, used an 820R for Rf and R2 turned out to be 68R for 0.42A emitter current). The RMS collector voltage still reaches 7V before clipping.

There are two key points though,

The Thevenin resistance of the bias circuit is less than 10R and is about 8.7R. This is essential because of the germanium transistor and its leakage properties.

Secondly, with the driver transformer and an appropriated driver device you can easily generate the appropriate drive power which is not insignificant, as you can see from the drive requirement specs.

I used a 22:8 R impedance ratio choke as the output transformer.

When the EF98 is used as the driver (handy as it runs off 12V) the required transformer has a turn's ratio of 23:1, the DCR of the secondary should be less than 1R and the primary less than 200R.

It is quite surprising how pleasant these class A stages sound, a lot like a valve radio...but devoid of hum, especially running off a 12V auto battery.

With a pair of OC16's in push pull you can get 17 to 20W with a 14V supply, which is very impressive for the day though you need a watt of drive power, which of course you could get from another OC16 in class A.

Also, one other interesting thing, using the EF98, it normally has a 10Meg Ohm G1 resistor. So you end up with an amplifier with two active devices EF98/OC16 & two inductive devices transformer/choke. With an input impedance of 10M, and output impedance of what your speaker is say 4R. It takes about 1 to 2V peak at the EF98 grid to get full power. That has to be one of the most spectacular power and impedance ratio transformations off all time, for just the two devices.
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Old 19th Oct 2019, 9:53 am   #27
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Default Re: Single-ended Transistor amps.

Hi Gents, if anyone is interested I can produce transformers and chokes for these amps.
I think I also have some nice nickle-iron lams for the driver transformers


Ed
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Old 19th Oct 2019, 4:22 pm   #28
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Default Re: Single-ended Transistor amps.

I've only skim-read this thread so apologies if I'm covering old ground.
I'd be tempted to avoid the old, collectable germaniums and use something like an MJE340 or even a BU208. This would enable the use of a fairly high HT and something like a small conventional audio output transformer.
I'll go back and read the thread properly now.....
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Old 19th Oct 2019, 5:13 pm   #29
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Default Re: Single-ended Transistor amps.

Thing is, I've got a load of old OC26/35/NKT404 Germanium power-transistors - which I'd honestly rather put to work than leave sitting in a component-drawer for another decade or two (only to have them thrown-out by the house clearance people when I finally snuff it).

I've also found a couple of AUY10 - which was one of the first early-60s TO3 Germanium power transistors that worked satisfactorily up to 10MHz or so (I believe it was originally designed for use as a ferrite-core-driver for computer memories). meaning I'm now wondering if the output transformer for any "single-ended transistor amp" should also include a tap to let me use it as an AM modulator?
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Old 19th Oct 2019, 11:28 pm   #30
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Gents, if anyone is interested I can produce transformers and chokes for these amps.
I think I also have some nice nickle-iron lams for the driver transformers
That is a nice offer.

I think this Germanium transistor Class-A audio output amp thing could be the next "Craze" that hasn't started yet because the exposure to amplifiers with this architecture in the audiophile fraternity is not widespread. Once the word got out about the simplicity, elegance and pleasant sounds etc of these amps, they might just catch on. Then that is really when you would have to worry about supplies & prices of germanium power transistors.

I custom made a stereo amplifier like this in the 1970's , with 2N441's on the output stage, hand wound output chokes. I had a black anodized chassis. The 2N441's were the Gold plated Motorola types on green anodized extrusion. (That it how that EF98/OC16transformer bracket in my radio got green anodized). It used more EF98's in the preamp.It was thought to be very cool at the time.
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Old 20th Oct 2019, 2:34 am   #31
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Default Re: Single-ended Transistor amps.

I think perhaps the biggest problem with small SE sand amps today will be speaker efficiency!! "In auld days" speakers were routinely 95 dB or so. Today a 12" speaker can handle 1000 watts RMS ( or so it is claimed) and maybe 82 dB/watt. That equates to some very big heatsinks !!!! not to mention battery grade cable to handle the idle current.


Just my take

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Old 20th Oct 2019, 8:27 am   #32
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Default Re: Single-ended Transistor amps.

Hi, yes a few of us made the WW "Bailey" amps with OC35's and custom made trifilar wound driver transformers. Pre amp stages were BC10*

They were a sweet sounding amp.

Ed
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Old 20th Oct 2019, 7:07 pm   #33
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by G6Tanuki View Post
I've also found a couple of AUY10 - which was one of the first early-60s TO3 Germanium power transistors that worked satisfactorily up to 10MHz or so (I believe it was originally designed for use as a ferrite-core-driver for computer memories). meaning I'm now wondering if the output transformer for any "single-ended transistor amp" should also include a tap to let me use it as an AM modulator?
Probably a subject for another thread, but I think the AUY10 was a remarkable device for its time.
They did work nicely in transmitters, certainly up to 40m with about 3 or 4W output from a 12V supply. They could be amplitude modulated using a small speaker matching transformer with a tap to modulate the driver stage. A suitable circuit can be found in a 1960's Tab book on simple transistorised transmitters. An audio amplifier using a pair of OC25's or similar, in a transformerless output stage, made a suitable modulator.
I have some AUY10's in the stores - might try building a clone of the one I had about 50 years ago!
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Old 20th Oct 2019, 10:35 pm   #34
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by m0cemdave View Post

Probably a subject for another thread, but I think the AUY10 was a remarkable device for its time.
I think the most remarkable to-3 germanium power transistor, especially for speed and high voltage applications, was the 2N3731, if you look up the specs it is quite an astonishing device by Germanium standards. It was designed by RCA for H deflection output stages in low voltage portable TV sets, as were other AU series transistors.
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Old 20th Oct 2019, 10:39 pm   #35
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by joebog1 View Post
I think perhaps the biggest problem with small SE sand amps today will be speaker efficiency!! "In auld days" speakers were routinely 95 dB or so. Today a 12" speaker can handle 1000 watts RMS ( or so it is claimed) and maybe 82 dB/watt. That equates to some very big heatsinks !!!! not to mention battery grade cable to handle the idle current.
I agree but I think these SE amps ideally should be kept in the range of 5 to 20W or below, and with modern speakers of that power rating, that is more than enough to fill a room with sound, bigger than that, go to push pull.
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Old 21st Oct 2019, 10:54 am   #36
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Default Re: Single-ended Transistor amps.

Quote:
Originally Posted by Argus25 View Post
The original AD149 made by philips or mullard had a beautiful bright nickel plated (on copper) finish nobody makes TO-3 transistors as beautiful looking that that anymore do they ? and the TO-3 case is obsolete. Or you could use a boring T0-220 cased epoxy silicon device.....not much of a contest is it?

Good point, yes, I have an AD149 somewhere, and yes it is beautiful


Quote:
Originally Posted by Argus25 View Post
Also, one other interesting thing, using the EF98, it normally has a 10Meg Ohm G1 resistor. So you end up with an amplifier with two active devices EF98/OC16 & two inductive devices transformer/choke. With an input impedance of 10M, and output impedance of what your speaker is say 4R. It takes about 1 to 2V peak at the EF98 grid to get full power. That has to be one of the most spectacular power and impedance ratio transformations off all time, for just the two devices.
To be fair, you can get an equivalent power gain from just ONE device, a power MOSFET operating in Class A... which you can use a 22MΩ gate-tie resistor with.

Quote:
Originally Posted by terrybull View Post
The John Lindsey-Hood original version 1969 class A uses 2n3055ís and sounds very good indeed. There is a version (for electrostatic speakers) with paralleled output transistors which produced 20 watts and a later version with + - power supply to remove the capacitor from the output.

I have built several of the original and the performance belies the component count.
JLH did produce a few rather good-performing circuits. Other people have done better technically, but that's not decrying what he managed to achieve. Not many people designed simple Class A transistor power amps. Not many want to! He did, and it has become legendary (a bit like the Mullard 3-3).

Incidentally, agree with Argus about sticking a transient suppressor across the output transistor, if operating into a transformer. It's an easy way to kill a transistor if not. A reasonable Zener (BZT03 series) is better than nothing!
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Old 21st Oct 2019, 1:35 pm   #37
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Default Re: Single-ended Transistor amps.

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To be fair, you can get an equivalent power gain from just ONE device, a power MOSFET operating in Class A... which you can use a 22MΩ gate-tie resistor with.
Yes, but to be very fair, both the EF98 and the OC16 are pre-1960 technology.

And, not only that, your power mosfet has an input capacitance many orders of magnitude higher than the grid capacitance of an EF98. The mosfet requires a lower Z source impedance to drive the gate, than with the EF98, if you want to maintain the high audio frequencies. It is is much more friendly looking into the grid of a pentode like the EF98 than into the gate of a power mosfet. The G1 of the EF98 just looks like your 10M resistor, the gate of the power mosfet looks like the 10M plus a parallel 500 to 5000pF capacitor in parallel, depending on the power mosfet type.

The G1 capacitance of the EF98 is 6.7pF and G1 is isolated by the screen grid action of the pentode construction eliminating the Miller effect too. If you can find a power mosfet that does that, I'll buy into your argument !

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Old 21st Oct 2019, 2:03 pm   #38
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Default Re: Single-ended Transistor amps.

For sure, thermionic and solid state devices have differences. There is no actual benefit in arguing one against the other. It is just a matter of designing different circuits around them. Use as many devices as it takes to do the job. Design for as good a performance as you feel you need. It's a hobby. It's a free country.

As far as designing audio amps is concerned, if you subscribe to a simple philosophy like 'Closest approach to the original sound' r envisage pieces of wire with gain, you can do as good a job as anyone is likely to need using any device you feel like. Bipolars, triodes, JFETs, Tetrodes, MOSFETs Pentodes, IGBTs. At a pinch you could do magamps or even get amplification from CRTs.

It ceases to be an issue of choice of devices and becomes one of how well appropriate circuitry has been designed.

We can choose to design an audio amplifier in all sorts of ways just for the hell of it and bring it to a satisfactory result.

A friend was developing amps using as few devices as possible that still worked well. So for a laugh I designed one that used as many as possible.... subject to each device doing something I could say was helpful. Great fun. Some of the former wound up being made at Linn, the latter at HP. A good time was had by all!

Remember, it's supposed to be fun. If it stops being fun, stop doing it.

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Old 21st Oct 2019, 2:13 pm   #39
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Default Re: Single-ended Transistor amps.

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Remember, it's supposed to be fun. If it stops being fun, stop doing it.

David
I agree with that. All sorts of tricks can be used to make "amplifiers" a good old one is a cmos logic inverter gate with a 1M resistor from its input to its output.

One I saw recently, quite creative I think, using a MAX232CPE serial driver IC, powered from 5V to make an audio amp. It uses the usual 1M for a feedback resistor around an inverter to turn a gate into an analog amp. Because of the flying capacitor step up voltage to +/- 8V it can produce a 14.5V peak to peak output signal. It was published in Silicon Chip page 99, Oct 2019, designed by Ariel Benvenuto from Argentina.
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Old 21st Oct 2019, 2:27 pm   #40
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Default Re: Single-ended Transistor amps.

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Remember, it's supposed to be fun. If it stops being fun, stop doing it.

David
Couldn't agree more!

In this case the 'fun' is building something-that-works-reasonably-well using the assortment of bits I've been discovering in my junkbox, rather than spending real-money [If I wanted to spend real-money I'd go to the nearest branch of Richer Sounds].

As to this particular amp, I fear I may be about to use a previously-unused component in it - a RS 15-0-15V 20VA mains-transformer. Heresy!
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