Tuac Amplifier modules

[Trevor]

Yes that is a better way of putting it! Not more reliable but less expensive to repair
fingers crossed that the tuac I have purchased is transformer coupled!
Regards Trevor

[Trevor]

unfortunatly the tuac i have purchased is a dc coupled unit of about 200 watts per channel
never mind i will carry on looking
regards trevor

[Lucien Nunes]

Not TUAC but for comparison, one of my electric organs has a pair of transformer coupled solid-state amps using only germanium PNPs. The design is much older than the silicon TUAC circuit and it was not built down to a price.

Each channel has four 2N2147s in cascode to give 40W into 8Ω from 35-0-35V. Claimed distortion is <1% at full power anywhere between 30Hz-15kHz. For organ use it's very important that the power and distortion figures hold all the way down to the very bottom end as this is where much of the continuous power is needed. The transformer would have been specced with this in mind and is AC coupled to the driver transistor.

Here's the circuit...

Lucien

[mikepitts]

Hi all, Name's Mike new to the forum, was just going round the web and googled T.U.A.C and came here !!

I was working as a motor mechanic and ran a mobile disco at the time, made up an amp with four of the 125 watt modules, Tuac was based in Mitcham where I went to buy the amps, I got to know the owner / designer think his name was Dave, they had all sorts of stuff being developed at the time, amps up to 500watts I done some work for dave on his Merc and he gave me the 4 x 125 watt amps for repairing his car, well it was like a child in a chocolate factory, they made 4 channel sound to light units and chasers, memory is going now but I think they sold their amps under the badge of baker, and norman Ithink, I have some photo's as well somewhere of my gear, nice to visit yesteryear, They also had a shop in tooting High street, and don't forget their mixers I had one where it was a touch sensitive pad for selecting pre fade listening, still enjoy old hifi a lot trying to find a pair of Ionofane plasma tweeters. hope this helps with the discussion.

Regards all, Mike.

[Trevor]

It certainly does help
I love the personel stuff ! I don,t suppose you have any remmains or diagrams on the tuac,s I am willing to pay for either
This is soley as a hobby I have studdied amplifiers for 40 years now and I do wonder if newer is better?
regards Trevor

[mikepitts]

Thing is Trevor, I love the sound of old speakers, I have all Rotel seperates with the amp being 380 watts rms per channel all transistor, speakers are Bowers & Wilkins 605,s all very good stuff and years ago 15 watts was all you needed, as I said in my last thread, as a kid always loved the sound of Tannoy's when I was 16 but never could afford them.

Now I just purchased some 1968 Bowers & Wilkins P2H speakers for £20, 4 foot tall 13" x 8" EMI speaker which is only 10 watts, should have a plasma tweeter but removed by previous owners but now looking to replace them, have popped a couple of good quality tweeters in for now, but the sound is wonderful so I am selling the 605's.

I have just bought 2 x Tannoy 15" Berkeley speakers and can't wait to try them just to see if they are as good as I remember.

anyway had a look last night and will try to scan a couple of photo's of some old TUAC stuff I bult all them years ago when I was a young lad.

Got to go to work now,

Rgards, Mike.

[Valvepower]

Hi Trevor,

I’ve uploaded/attached some pictures of a TUAC module I have in the workshop. I’ve not got around to copying out the schematic from the PCB though!

I got the module to help the research into the design of transformer and low impedance drivers for transistor amplifiers as part of a research project.

I’ll post a bit more tomorrow.

Regards
Terry

Originally Posted by Trevor

In My youth
I used to see often the Tuac modules advertised in the electronics mag,s
Does any one out there have any information re these as I most keen to study the circuit
The unusual feature of these amplifiers was that they used a driver transformer
best regards for now
regards Trevor

[Lucien Nunes]

Hmm, similar but different. 1975 if you believe the caps, transformer 1979 maybe. Clearly the unit is later than my 1973 100W job and in that time they've made the transition from single to split supplies and tidied things up a little. Surprising that they are using bare resistance wire for the emitter resistors in place of the 'proper' components in mine. Perhaps that's what mine had originally.

Seeing the underside of your board solves the mystery of the 5-pin transformer in mine - there's another leadout hidden under the bobbin soldered directly to the board.

Would be interesting to know what the LF performance of yours is like.

Lucien

[Trevor]

Valve power
That is superb just what I am after the schematic would be the icing on the cake !
This is to me a nearly forgotten art in audio amplifier design
A simple way for 100 watts with only 5 active devices great
best regards trevor

[Trevor]

Re the earlier post and 5 active devices I am forgetting that the 100 watt as shown has 2 pairs of output devices in parrallel making 7 devices in all
I feel certain that this is a later version using high voltage devices as and when they became available
I feel certain that the earlier 125 watt version used 4 2n3055 devices in series with a driver transformer that had 5 windings in total thus enabling it to use +/- 50 volt rails
I was suprised to see that the version shown by valve power had split rails as I remember them being capacitor coupled to the spk
but I suppose they evolved as time went on
best regards for now Trevor

[Valvepower]

Hi Trevor,

I’ve more or less finished copying the circuit, but before posting it I just want to clarify a few parts of the circuit as they seem strange. I hope to scan and post it tomorrow.

From memory the TUAC modules used 2N773 or 2N6254 - I may be wrong though. The 2N773 was used in latter HH amps and the 2N6254 was used in the MM AP360. The transistors in the unit I have here are SP507 - they have typical RCA markings. At that point TUAC may have had a high enough quantity for RCA to mark them up with their own part number?

Looking at the values of the components in the feedback circuit I’d estimate it has an input sensitivity of approximatly 50mV, which is quite sensitive for this type of module.

I remember a friend using a pair of these power amplifiers in a mobile disco in 1975. Sadly I can’t remember that much about them, but I do remember those sounding rather good with a pair of folded horn cabinets and compression drivers.

Regards
Terry.

[Valvepower]

Hi,

I’ve attached the circuit for the TUAC TP125. You’ll have to excuse the “back of a fag packet” drawing style!

I did have to double check the resistors in the feedback network as this appeared strange at first sight, but it is right though. I feel the extra part positions (fitted with resistors) could be replaced with capacitors for the inclusion of some sort of equalization network on special units?

I also had to double check the 5R6 in parallel with the 2K7 in series with the 22uF in the bias network feeding the base of the first transistor.

I’ve also included the circuit for Bailey and Rogers Ravensbourne amplifiers.

Terry.

[Valvepower]

I've attached a circuit in PDF format, which could be better than the JPEG above.

Terry

[roffe]

The feedback circuits don't make sense,I'm afraid.Well not to me,anyway.Otherwise it looks OK.
The first simply has to be a DC only feedback.As it is now,it's both AC high gain and DC.I wonder...
The overall feedback gives a gain of roughly 770 times.Again I wonder..
What disturbs me are both 5R6.They don't make sense.
Please take a look again!

[Herald1360]

Doesn't the first feedback loop simply set the dc conditions for the front end and driver, whilst leaving the AC gain open loop? The overall loop (AC only thanks to the transformer) then sets the signal gain of the entire amplifier.

If the overall amp gain is x770, that's not far out for around 50mV sensitivity driving the output to clipping on a 40v rail.

[Trevor]

Valve Power many thanks for your efforts re the schematic
can you advise on the dc resistance of the windings as these form part of the bias scheme for the output devices
I would also like to find out re the turns ratio primary to secondary
The Bailey schematic is also very much appreciated I have a rather poor example of the bailey in my collection which was given to me when I went to collect a Rogers Ravensbourne
The Bailey was a kit made by hart electronics and despite a few attempts to contact hart I have not untill now made any progress but with your help i have made a great leap forward
Now all I need is to locate a source of transformer parts or some one who can make me a few transformers any Ideas ?
I have had a few thoughts re the driver stage using +/- 50 volt supplies I calculate the driver stage will pass 100mA and disspate about 4-5 watts
I agree that the feed back network is is a little odd however the inner loop might have alimited amount of feedback in order to stable-ise the front end
the outer loop might have been done with so many resistors in order to adjust the gain of the power amp by shorting out the appropriate part of the network
input impedance is about 10kso 50 mv and 10 k seam to be odd values but that may just be the starting point
best regards Trevor

[Valvepower]

Hi,

I’ve attached closer shots of the top and bottom of the PCB if it helps.

Anglia transformers the company who made the driver transformer in my TP125 are still in business. See the web site link below:

http://www.angliatransformers.co.uk/index.html

As far as the 5R6 is concerned I’ve seen the use of low value resistors in place of links or zero ohm resistors, where it wouldn’t affect the circuit conditions!

I agree with Herald1360, there are two feedback loops:

The local DC feedback around the driver stage, which is decoupled for AC signals via the 22uF capacitor, the 5R6 will introduce some AC feedback but not that much seeing the value of the resistor from the emitter of the 2n3055 is 2K7. The input sensitivity is set by the main AC feedback, taken from the output to the emitter of the first transistor.

Another observation is the lack of a Zobel network across the output.

I’ve also attached an article by J. Sugden on Class A amplifiers from the Nov 1967 hi-fi News – the 3rd paragraph page 591 has something on transformer drivers.

Regards
Terry.

[Trevor]

I have been in contact with Anglia transformers and they will shortly advise re the price of a new driver transformer
I will keep you all posted re price etc
I intend to build a stereo power amplifier and will post as it goes on
regards Trev

[Valvepower]

Hi Trevor,

Thanks for the update.

I’d be interested to know how much Anglia are going to charge for a pair of transformers.

Terry.

[Valvepower]

Hi Lucien

I’ve just had another look at the module in your amplifier and I feel it may be a Saxon amplifier module, they also advertised in PW and PE in the 70’s. This was there heavy duty module using a driver transformer, the rest of the range used DC coupled quasi complimentary circuits.

I remember one of these amplifiers lying around the workshop I worked at in the mid 70’s.

Terry.

Originally Posted by Lucien Nunes

Hmm. TUAC. Brings back memories.

I've attached a couple of pics of a Baker Major 100W amplifier. This PA looks like a close relation of the ones in the PE ad. IIRC there's no mention of TUAC on the PCB but I've seen other TUAC PCBs with their name in the foil somewhere.

Lucien