HH TPA series Power Amplifiers.

[McMurdo]

As has been intimated already, these old power amps often have no DC protection and can and do fail DC. So be wary of connecting your vintage and beloved hifi speakers to them unless the amp is in tip-top condition, the amp will fry them in milliseconds.

HH were briefly part of Carlsbro and also made big-ish amps for fairground rides, I've just repaired one. They also made amps for the BBC presenter cubicle monitor speakers as used in local radio stations. They later became part of Headstock BLT Industries who also make 'Laney' equipment.

[Trevor]

I have repaired the faulty Tp100 amplifier and will when I have drawn the schematic post it
The main problem was an open circuit 1k resistor in the bias chain
also a npn transistor in the protection circuit was faulty
The output standing voltage was 22 mV when loaded with an 8 ohm speaker Not bad for a simple amplifier there is no trace of crossover distortion on sine wave testing I am yet to listen to them on music
Calculated output is 240 watts rms into 8 ohms but as the amplifier has only 1 pair of output transistors is suspect that the unit was designed for 15 ohm loads
This amplifier has done in effect something that I have been considering for the last 10 years or so it is very simple but delivers high power with a low component count all in all a good effort
regards Trev

[teetoon]

Hi Trevor, most interesting. When you say the faulty amp, was it not working at all? I did have them running as a stereo pair it was just the distortion that stopped me using them. I assume the 1k resistor is one of the wirewound green ones? I might get out the remaining 100 and have another go at it now after your report. What about the different value resistors I mentioned? Have you changed anything there?
The output in the TPA series was always quoted into 15 ohms but the power supply can clearly put out more as you say. I tried them with and without the output transformer connected but couldn't detect any audible difference. I assume it imposes no load without anything connected to the secondary?
All in all then, not a bad design.

David.

[Trevor]

Teetoon I did not mean to say or implythat the faulty amp was not working at all! My standard procedure with any amplifier is to measure the supply volts and dc offset before I connect the speaker load The bad amp had a dc offset of 36 volts
However when I connected a dummy 8 ohm load the offset fell to 2.5 volts and the fault current was about 80 mA! This then made me suspect the bias network on checking I found a 1k 6 watt resistor open circuit on replacing this the offset fell to 0.2 volt open circuit and 20mV when the load was re connected
I then connected a signal source and injected a 1000hz tone the result a lovely clean sine wave I did not do this before changing the faulty resistor
Regards Trev

[teetoon]

Hi Trevor, That's interesting, I remember they both sounded great at first and gradually over a period of weeks there was an amount of distortion so it must have been that resistor. I suppose it's hardly surprising given the age of them and the use they must have had. I look forward to your verdict on their sound quality.
David.

[Trevor]

Please find attached the schematic re the power amplifier
I have tried to draw it a clearly as I can but I know its a weakness of mine re handwriting and drawing etc
I am waiting for some new iec mains sockets and cannon adaptors before I can post any comments re sound quality however on my workshop speakers they are very quite and they look good on the scope !
You sometimes have a good feeling re a design and these fill me with lots of good feelings here!!!
I love the fact that if something goes wrong theydont blow everything to bits
Any Questions and comments will be well received I can not thank Teetoon enough for making these available to me
regards Trev

[Valvepower]

Hi Trevor,

Nice job.

There are aspects of the driver circuit that resemble the Bailey amplifier from 1966.

I can’t see any parts which compensate for temperature in the output stage? I can see the diodes in the protection circuit but can’t see any temperature compensation parts.

Un earthed an late 70’s MA100 with a black front panel.

Terry.

[Trevor]

Terry,
That's the great feature, by using a low impedance darlington you can do without any temperature compensation. The driver transistor alone feeds the load and then as current demand goes up the main outputs come on and assist, aka the crown dc 300 ab/b.

About thirty years ago I made 10 off 250 watt into 4 amplifiers for a local disco/heavy rock venue. I thought at the time that I had made a breakthrough in amplifier design not knowing anything about Amcron etc.

Regards, Trev.

PS Lets have a picture of your Ma100.

[roffe]

Trevor-hate to pick on you again,but there are some things to be corrected in the schematic.Better to do it now than causing problems later.
There is a direct connection between output and input of the power amp.
There is no connection between pre and power.
Feedback seems OK if it's rerouted to "out".
What are the coils and the 2N3055 in the pre?Protection?Only 1 BC107 as voltage amp?

[Trevor]

Roff the coils are the driver transformer windings
the input is there look a little closer
The fault is mine I can not draw for toffee nuts but its my best attempt
and yes only 1 bc107 as the voltage amp the main gain is due to transformer coupling
regards Trev

[Trevor]

The first coil in series with the capacitor and in parrallel with the capacitor is a reed relay coil that is normally closed. The contacts of that relay are across the primary of the driver transformer this is a form of switch on muting!
The coil in the emitter of the driver transistor is the primary of a driver transformer the secondaries are connected to the driver transistors
The feedback is from the output to the emitter of the bc107
regards trev
regards

[roffe]

A few hours after writing I realised that it must be an interstage transformer.Silly me!Well,now it makes sense.Thank you!

[Valvepower]

Hi Trevor,

I’ve attached a zip file with some pictures of the MA100 and the original circuit diagram. This unit was made in May 1981.

The circuit diagram is an original HH diagram, and dates back to when I was servicing HH amplifiers in a local music shop, who were HH dealers at the time. Note the changes to the resistor values on the pre-amp circuit; I vaguely remember these were changed to reduce input overload for a particular customer.

Regards
Terry.

[julie_m]

BBC Radio Derby used to have some shoebox-sized mono amps by HH, feeding monitor speakers around the station. My dad, who was a presenter there at the time, borrowed one to use for a street party (probably Charles + Diana's wedding, or maybe Andrew + Fergie). It was certainly capable of some serious volume. Don't remember a model number, unfortunately

[Valvepower]

Hi AJS,

I’m not totally sure, but I think the model number for the HH BBC monitor amplifier is AM8/12. It has switchable bass lift EQ and was designed to drive the LS3/5a speaker.

Regards
Terry.

[puremilk]

I need the circuit diagram for TPA100D, if anyone could help?

Thanks.

[Kala_12]

Hi puremilk,

Did you ever get that circuit diagram for your TPA 100D?
I still have a 'handbook' from one of the pair of these amps I purchased 35+ years ago. It contains the circuit diagram, but unfortunately NOT the values of the components. If this is of any use, I can scan it and send it to you.

Regards,
Stuart

[Trevor]

I would love to see a copy of the circuit diagram.

Regards,
Trev.

[Jeff D**]

I would like a copy of the 100D circuit too please.

Regards,
Jeff

[Kala_12]

Hi Trevor,
Here you go. If would like anything else (overview, installation, specs), feel free to ask.
Stuart.