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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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18th Oct 2019, 7:07 am | #21 |
Heptode
Join Date: Apr 2003
Location: Cornwall, UK.
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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. I’ve listened to many other HiFi amps over the years, but the Linsley Hood holds its own with ease! I’m looking forward to seeing what you can cobble together! Cheers. SimonT.
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18th Oct 2019, 6:36 pm | #22 |
Dekatron
Join Date: Apr 2012
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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? |
18th Oct 2019, 6:52 pm | #23 |
Octode
Join Date: Jun 2006
Location: St Osyth, Nr Clacton, Essex, UK.
Posts: 1,482
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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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18th Oct 2019, 9:31 pm | #24 |
Rest in Peace
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Location: Matlock, Derbyshire, UK.
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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. |
19th Oct 2019, 12:31 am | #25 | |
Octode
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Location: Evesham, Worcestershire, UK.
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Re: Single-ended Transistor amps.
Quote:
Greg.
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19th Oct 2019, 1:22 am | #26 |
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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. Last edited by Argus25; 19th Oct 2019 at 1:38 am. Reason: various typos |
19th Oct 2019, 9:53 am | #27 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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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 |
19th Oct 2019, 4:22 pm | #28 |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
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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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19th Oct 2019, 5:13 pm | #29 |
Dekatron
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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? |
19th Oct 2019, 11:28 pm | #30 | |
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Re: Single-ended Transistor amps.
Quote:
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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20th Oct 2019, 2:34 am | #31 |
Banned
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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 Joe |
20th Oct 2019, 8:27 am | #32 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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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 |
20th Oct 2019, 7:07 pm | #33 | |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,223
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Re: Single-ended Transistor amps.
Quote:
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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20th Oct 2019, 10:35 pm | #34 | ||
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Re: Single-ended Transistor amps.
Quote:
Quote:
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21st Oct 2019, 10:54 am | #35 | |||
Dekatron
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Location: Lynton, N. Devon, UK.
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Re: Single-ended Transistor amps.
Quote:
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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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21st Oct 2019, 1:35 pm | #36 | |
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Re: Single-ended Transistor amps.
Quote:
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 ! Last edited by Argus25; 21st Oct 2019 at 1:43 pm. |
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21st Oct 2019, 2:03 pm | #37 |
Moderator
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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. David
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21st Oct 2019, 2:13 pm | #38 | |
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Re: Single-ended Transistor amps.
Quote:
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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21st Oct 2019, 2:27 pm | #39 | |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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Re: Single-ended Transistor amps.
Quote:
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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21st Oct 2019, 5:19 pm | #40 |
Dekatron
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Re: Single-ended Transistor amps.
i have few traniie driver tfmr's if you need any for your amp build, look forward to seeing how you get on. apologies fore bad typing can't use right hand, whacked this morning shifting stage gear..
andy.
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