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Old 29th April 2016, 09:47 AM   #1
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Default RH slayer

Hi all,
I recently built a very simple little amplifier which I though would be worth sharing. People might know that I have been a violent critique of the whole RH amp concept because of its inherent design flaws. I decided to see if I could design a simple cheap Schaded amplifier based upon a sound understanding of the principles of Schaded feedback. This is what I came up with.

Things to note are:

-it uses a Switch mode Power supply to generate a 12V regulated supply for the heaters and to feed the +B power supply. One of these can be had for 10.00 from:
AC 85 265V TO DC 12V 8A AC DC 50 60Hz Switching Power Supply SMPS Moudle Board | eBay

-the B+ is generated by using a boost supply which generates 230V DC. This can be had for 10.00 from:
150W Inverter Battery DC 12V TO DC 220V Voltage Step UP Boost Converter Module | eBay
At the power levels this unit is operated at there is no need for the fan which has an slight hum to it.

-the valves themselves are Russian dual tetrodes which I got from an eastern european suppler 12.00 (inc. P+P) for a quad.

-the toroidals used for the output need only be small 40-50VA units and can be had for about 15.00 a throw.

-the only really expensive item is the 12L8GT which is an obscure twin pentode valve which I used because I wanted a three valve design. Its quite expensive and generally only available from America. I will be working up a design which uses either the 6AU6 or the even cheaper EF91 which can be had for peanuts.

It should be possible to build this unit for less than a 100.00

The bias adjustment is used to ensure that the voltages at the top of the tetrode plates are exactly equal, which ensures that the DC standing current in the transformer are equal on both sides.
The pot on the IRF710 is used to set the voltage drop across the driver 10K plate load to about 80V.
I built the bias adjuster on a little perf board and used a salvaged 8+8V transformer. Any small dual secondary transformer of low voltage will do.
I also used tag strips to mount most of the components on.

The unit will produce about 5W of power per channel. This has an overwhelmingly second harmonic distortion profile and it measures very like a Triode SE design. It is not an ultra low distortion design and was not designed to be such. I am listening to it on my secondary system and it matches up extremely well to any of my other designs.

Shoog
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Old 29th April 2016, 09:59 AM   #2
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Missed a smoothing cap on the bias supply
The 670R resistor on the screens of the dual tetrode has enamelled wire wrapped round it 10 times and is soldered to each lead of the resistor. This offers no resistance at frequencies below RF, but kills any tendency to RF instability. The 10R resistor on the plates should be soldered right on the pins of the valve base for the same reason.
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Last edited by Shoog; 29th April 2016 at 10:08 AM.
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Old 29th April 2016, 10:01 AM   #3
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PP with overwhelmingly second harmonic distortion profile? What might be the explanation for that?
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Old 29th April 2016, 10:02 AM   #4
DF96 is offline DF96  England
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Quote:
Originally Posted by Shoog
I have been a violent critique
Maybe you meant to say 'critic'? Unless, of course, you are not a person but an argument personified!
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Old 29th April 2016, 10:05 AM   #5
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@shoog - may be not "violent" either, perhaps just a vocal critic. You have always come across as quite polite, unlike AK.
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Old 29th April 2016, 10:12 AM   #6
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The reason you get overwhelmingly second harmonic distortion is because the driven tetrode is Schaded to the driver which turns its character into a pseudo triode. the second tetrode is running as a pure tetrode which means it is slaved to follow the signal fed it by the driven tetrode - which it does very well. Secondly the current through the transformer is unbalanced by the bleeding off of the driver plate current - which in turn contributes to a second harmonic character.

Shoog
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Old 29th April 2016, 12:39 PM   #7
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But isn't the push pull operation suppose to cancel the even harmonics?
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Old 29th April 2016, 12:43 PM   #8
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I believe only if the behaviour of the two elements are identical.
Take for example a not very well matched pair of triodes in PP, it will have a higher second harmonic distortion than one which has a matched pair.

The simulation results confirm this behaviour. In this design the two elements are extremely poorly matched.

Shoog
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Old 29th April 2016, 12:52 PM   #9
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I asked the same thing on my own thread Difference Between Self-Inverting Push Pull and Anti-Triode?
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Old 29th April 2016, 02:04 PM   #10
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Cool, guys, I will have to breadboard one and check it out for myself, although I must say that I am not a fan of the self-split output stage...
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