OTL - The mistrusted valve amplifier?

[Firebottle]

Any regular on AoS will know of my love of OTL valve amps (Output Transformer Less for the uninitiated).

I have heard, as probably have most valve aficionados, of scare stories of OTLs going faulty and blowing speakers, though a lot of DIY'ers still make them.

For me the sound is sublime but there are limitations to speaker compatibility.
I'm currently thinking along the lines of a design for a 10W low cost, easy to use and safe OTL:

Low cost One of the areas where a cost saving is made is of course the absence of the output transformer, always a big expense to achieve great performance with a traditional valve amp.
With suitable connections between them the power supply requirements can be met with two off the shelf transformers, cost £76.

Valve choice is always going to have an impact but there are some low cost options around at the moment. 2 off 6080 per channel will give 10W and there are some available for £6 each, and using 6J6 for input and driver stages will only cost $3 per valve.

Ease of use Some people may find the requirement of having to bias an amplifier a chore or even a PITA, so I am working on an auto-bias scheme that will need no adjustment at all.
I haven't come across an OTL that offers this as yet.

Loudspeaker protection Nowadays it is very easy to incorporate a loudspeaker protection circuit, indeed foolish not to. A small kit from the far east can be had for under £10.


Of course one of the greatest drawbacks of any OTL is the diabolical efficiency, far worse than even a Class A amplifier, but as they say 'you can't get something for nothing'.

Your thought chaps?

:cool: Alan

[Reffc]

Good idea Alan. What's not to like? Low cost, speaker protection, autobias (cathode follower?).
Are you thinking of these as kits or ready-builts?
What sort of bandwidth and distortion are we talking at say 90% power?

[Radford Revival]

Some kind of DC offset servo would be advantageous I'd imagine. You'll probably want apply a fair amount of feedback to get the output impedance down to something usable.

[Ammonite Audio]

All of the amplifiers that I use are OTL, but none of them use valves ;)

[awkwardbydesign]

Are you talking about a design or a finished amp? I would be very interested in making one myself.

[walpurgis]

Your OTL project sounds a great idea. 10 watts is enough for those of us who use efficient speakers. Go for it I say! :)

[Gordon Steadman]

I, of course, might be said to be somewhat biased.

However, having heard Alan's latest OTL driving the Teufels to somewhat generous levels in my big lounge, I can affirm that it sounds bloody gorgeous. With only one valve and 10 watts it clipped a bit but the Teufels are not over efficient and its a big room. With the two valves and 20 watts it was more than adequate for realistic sound levels.

Alan knows what he is doing.

[struth]

What kind of sensitivity would be required for speakers Alan?

[Ali Tait]

I'd say ease of load will have more impact than outright sensitivity, OTL's don't like a reactive load.

[Firebottle]

@ Richard> It's a bit of both at the moment. The true amplifier part is all proven as it's the same as my system OTL. The simpler power supply and auto bias are still on paper at the moment, I'm going to prove the auto bias circuit before I post the circuit.

@Paul> If the latest OTL is anything to go by the power bandwidth is amazing, 10Hz to 80KHz with the modified Tim Mellow circuit. Can't quote you a distortion figure but its very clean.

@Grant> It really depends on how loud you want it :scratch: I would surmise that 90dB would probably suit for most requirements, plus they say valve watts appear louder, don't ask me how :mental:

:cool: Alan

[The Black Adder]

I know that Croft OTL's sound amazing. A friend of mine runs one and it's superb. Protection circuits are a must of course especially with expensive or rare speakers. I don't really know much about them in comparison with PP amps as I've never done an A-B comparison. Some say they are just a bit too twee on the sound... Maybe someone can give a more precise opinion of experience.

[Ali Tait]

Soundstaging is amazing on suitable speakers, best I've heard from any kind of amp. They can be very speaker dependent though, I've heard OTL's sound very ordinary through speakers not suited.

[hifi_dave]

Back in the day, we sold bucket loads of Croft OTL Series 3 and they were great. Clean, clear, fast and dynamic, they easily drove all the speakers we had on demonstration except for the Apogee full range ribbons.

The current Croft Series 7 power-amps are very close in sound quality, at a fraction of the price.

[NRG]

I may be way off base here but I've never heard low cost and OTLs mentioned in the same breath! The terms are usually mutually exclusive! Im sure it can be done but will it sound any good? Apart from working out to handle a valve failure as mentioned above there's a need to parallel sufficient o/p valves to lower the o/p impedance before wrapping it all up with feedback. So you need more gain from the preceding stages... More o/p tubes = more current draw and heat generation and this pushes the cost up further. And as Ali says even when well thought out OTLs tend to be very dependent on the 'speaker they drive. Hats off to you though if you can make it work...

[Ali Tait]

I liked the single ended OTL I heard once. Very limited power though, so speaker choice is limited. Great sound though.

[Jimbo]

Back in the day, we sold bucket loads of Croft OTL Series 3 and they were great. Clean, clear, fast and dynamic, they easily drove all the speakers we had on demonstration except for the Apogee full range ribbons.

The current Croft Series 7 power-amps are very close in sound quality, at a fraction of the price.

The Series 7 is an OTL without all the potential valve OTL problems and as you say Dave they do sound very close to valve OTL amps even though they use MOSFETs.

[Gordon Steadman]

I know that Croft OTL's sound amazing. A friend of mine runs one and it's superb. Protection circuits are a must of course especially with expensive or rare speakers. I don't really know much about them in comparison with PP amps as I've never done an A-B comparison. Some say they are just a bit too twee on the sound... Maybe someone can give a more precise opinion of experience.

Twee!!! Far from it. Full bodied, great bass and very clear and precise treble. The sound is tangible and real. I've only heard two and they have both been made by Alan so how they compare with others, I have no idea.

[struth]

@ Richard> It's a bit of both at the moment. The true amplifier part is all proven as it's the same as my system OTL. The simpler power supply and auto bias are still on paper at the moment, I'm going to prove the auto bias circuit before I post the circuit.

@Paul> If the latest OTL is anything to go by the power bandwidth is amazing, 10Hz to 80KHz with the modified Tim Mellow circuit. Can't quote you a distortion figure but its very clean.

@Grant> It really depends on how loud you want it :scratch: I would surmise that 90dB would probably suit for most requirements, plus they say valve watts appear louder, don't ask me how :mental:

:cool: Alan

Might well be suited to these beovox,s I've just obtained then. Would be a nice little system me thinks

[Firebottle]

I may be way off base here but I've never heard low cost and OTLs mentioned in the same breath! The terms are usually mutually exclusive! Im sure it can be done but will it sound any good?.

Hi Neal,
Thanks for the encouragement, I have been giving this a lot of thought, in reality along the lines of a self build. That way you can spend as much or as little on the casework, always a bit of a sting with a commercial valve amp.
As I said with an educated choice of what valves to use (trawling the web for realistic pricing) plus a simplified approach to the power supply requirements can save big time.

The low power OTL I use on a day to day basis probably cost less than £200 in parts, but that was with a donor case from a defunct piece of test equipment.

:cool: Alan

[awkwardbydesign]

@ Richard> It's a bit of both at the moment. The true amplifier part is all proven as it's the same as my system OTL. The simpler power supply and auto bias are still on paper at the moment, I'm going to prove the auto bias circuit before I post the circuit.

@Paul> If the latest OTL is anything to go by the power bandwidth is amazing, 10Hz to 80KHz with the modified Tim Mellow circuit. Can't quote you a distortion figure but its very clean.

@Grant> It really depends on how loud you want it :scratch: I would surmise that 90dB would probably suit for most requirements, plus they say valve watts appear louder, don't ask me how :mental:

:cool: Alan
Thanks Alan. I'm using a Stereo Coffee LDR pre at the moment (I'm hoping it's a keeper; promising so far), so power amp gain and input impedance would matter. The speakers are Troels Gravesen Jenzens, apparently 89dB and an easy load.

[anthonyTD]

:)
I'd say ease of load will have more impact than outright sensitivity, OTL's don't like a reactive load.

[Gordon Steadman]

I'd say ease of load will have more impact than outright sensitivity, OTL's don't like a reactive load.

Alan's are brilliant with the Quad57s, both he and I use them.

[Ali Tait]

Aye, as I said, they like speakers that are an easy load. Statics may have a sharp dip in impedance in the treble, but they are otherwise generally an easy load to drive.

The 57 is a lovely speaker, used to have a pair myself.

[loo]

I'd say ease of load will have more impact than outright sensitivity, OTL's don't like a reactive load.

Depends on the OTL in question , a pair of these would drive the flying Scotsman:eyebrows:
mind you with 84 output valves and 42 driver valves per stereo pair a revalve could be pricey
http://i1308.photobucket.com/albums/s612/loonieboo1/60ae2c31-fead-404f-afb8-7df8182267a9_zps9d60667b.png (http://s1308.photobucket.com/user/loonieboo1/media/60ae2c31-fead-404f-afb8-7df8182267a9_zps9d60667b.png.html)

[Ali Tait]

Like the sign in the film said, "Speed is just a question of money, how fast do you wanna go?" :-)

[loo]

Hi Alan, not another 10 watt per channel OTL, you know what we really want is a hair shirt 200 watt monster that dims the lights on switch on:eek:
Only joking ,sounds like a great idea and a great way for sceptics to dip into the world of OTL's.
BTW Bruce R's amps are all auto bias now I believe
Cheers
Paul

[Mr Kipling]

It's got ta have lights! It's got ta have lights! The full Blackpool rig for me.

The 6C33 appears to be popular, but some reckon it can be a bit flakey. Have a bit of thing for the 813/FU13. I'd want something that looks a bit - flash.

Oh, and meters. Got ta have meters.

http://img.tapatalk.com/d/14/10/14/dyre8usu.jpg

http://img.tapatalk.com/d/14/10/14/re9aqy2u.jpg

http://img.tapatalk.com/d/14/10/14/zygajeny.jpg

These look nice.

[Ali Tait]

I used to have an 813 amp, sounded good driving my statics. Have at leasr a dozen pairs of valves in the cupboard.

[Mr Kipling]

I used to have an 813 amp, sounded good driving my statics. Have at leasr a dozen pairs of valves in the cupboard.

Looking for one with a fried mains. Must be a few out there by now.

[Firebottle]

First step in testing the auto bias circuit, assembling the variable HT power supply.

As usual it's making use of what I've got kicking about. Started life as a gen purpose 0-12V AC / 0-16V DC psu. I've taken out the low voltage rectifier and added a 9V to 120-0-120V transformer to give me a max output of about plus and minus 155V DC.
Adjustment is via a variac transformer on the input so I've got full control of the final output voltage. Pic for all you inquisitive folks:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/P1040630.jpg

Cheers,
Alan

[Ali Tait]

Looking for one with a fried mains. Must be a few out there by now.

Sorry missed this, yes, I daresay, that's what happened to mine!