Remember These?

What people are working on at the moment
Wolfgang
User
Posts: 174
Joined: Mon May 07, 2018 3:08 pm

Re: Remember These?

Post by Wolfgang » Mon Nov 11, 2019 3:11 pm

Just thinking about the whole OTL thing. Mine, and all the rest of the single ended OTLs out there (apart from the only inverted OTL in the world) may be “output transformer less” but they are certainly not direct-connected to the speakers the way totem pole OTLs are, so the quality of the output cap will affect sound quality, just like an output transformer does, so FFS, bypass your output cap with summat decent.
Do you consider the psu caps in series with the output cap regarding the audio signal?
In this case a cap multiplier with very little capacity caps like I use with the InvOTL or a HV reg psu like the Maida reg. for the output stage (keeping the TL738 as CCS) would improve the SQ.
How did you calculate the size of your output cap?

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Mon Nov 11, 2019 5:02 pm

Wolfgang wrote:
Mon Nov 11, 2019 3:11 pm

Do you consider the psu caps in series with the output cap regarding the audio signal?
In this case a cap multiplier with very little capacity caps like I use with the InvOTL or a HV reg psu like the Maida reg. for the output stage (keeping the TL738 as CCS) would improve the SQ.
How did you calculate the size of your output cap?
Yep I did consider that in a single-ended amplifier, the power supply is in series with the audio signal, and the 2200uF output stage PSU caps are also now bypassed.

The output cap wasn’t calculated, it was all I could find in a box after I’d blown up one of the the previous caps on the breadboard, when a bad 13E1 arced over. Using the DC resistance of the speaker load, in this case 7R, with the 6800uF I had, got me a corner frequency of about 3Hz. “That’ll do” I thought. I’m aware that speakers do not act like resistors, but you have to work from somewhere.

Unfortunately, my OTL is very much the “poor relation” to both yours and Ray’s builds. I simply don’t have the funds to buy, or build HT regulators or DC heater modules. The cost of heatsinks is a problem for me, but thanks for the suggestions. :)

My OTL is basically, made up of bits and pieces that I’ve picked up here and there. The triple power supply arrangement is about as far as I can go with this OTL. That it even works, and better yet, sounds as good as it does, never ceases to amaze me. I have to smile very time I turn it on.
On every tree there sits a bird,
Singing a song ...of love...

User avatar
Ray P
Social outcast
Posts: 3596
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Mon Nov 11, 2019 7:17 pm

Cressy Snr wrote:
Mon Nov 11, 2019 5:02 pm
Yep I did consider that in a single-ended amplifier, the power supply is in series with the audio signal, and the 2200uF output stage PSU caps are also now bypassed.

The output cap wasn’t calculated, it was all I could find in a box after I’d blown up one of the the previous caps on the breadboard, when a bad 13E1 arced over. Using the DC resistance of the speaker load, in this case 7R, with the 6800uF I had, got me a corner frequency of about 3Hz. “That’ll do” I thought. I’m aware that speakers do not act like resistors, but you have to work from somewhere.

Unfortunately, my OTL is very much the “poor relation” to both yours and Ray’s builds. I simply don’t have the funds to buy, or build HT regulators or DC heater modules. The cost of heatsinks is a problem for me, but thanks for the suggestions. :)

My OTL is basically, made up of bits and pieces that I’ve picked up here and there. The triple power supply arrangement is about as far as I can go with this OTL. That it even works, and better yet, sounds as good as it does, never ceases to amaze me. I have to smile very time I turn it on.
My usual power supply modules are made up of smaller value caps in parallel.

Steve, hopefully this week I'll get to run the 13E1 filamants off some simple regulated supplies - I want to check the heat dissipation on the heatsink I have in mind. If it works out I'm happy to give you a couple of the PCBs which I think you'll be able to populate for less than £20.

Also, if you can give me a little time I can loan you a Maida regulator board configured for around 150V if you're interested in trying it on your input/driver section? If it works for you we can work something out just to cover the cost of the parts.

Ray

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Mon Nov 11, 2019 7:59 pm

Sounds good to me. Thanks Ray.
On every tree there sits a bird,
Singing a song ...of love...

User avatar
Ray P
Social outcast
Posts: 3596
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Mon Nov 11, 2019 9:33 pm

Cressy Snr wrote:
Mon Nov 11, 2019 7:59 pm
Sounds good to me. Thanks Ray.
I'll PM you when I've checked them out Steve. Here's a picture of one of the little reg boards, just 55mm x 40mm...

Image

This one's for 6.3V, for 26V the cap is a lot bigger.

The Maida reg board is a bit bigger, 75mm x 80mm...

Image

Wolfgang
User
Posts: 174
Joined: Mon May 07, 2018 3:08 pm

Re: Remember These?

Post by Wolfgang » Tue Nov 12, 2019 3:08 pm

Also, if you can give me a little time I can loan you a Maida regulator board configured for around 150V if you're interested in trying it on your input/driver section? If it works for you we can work something out just to cover the cost of the parts.
For true test Steve would need a transformer of at least 177V AC (50VA is enough for the test, but the more the better) which would give the needed 190V DC for a perfect voltage swing of the 6H30 in both directions. The Maida reg. would need to be set up for this (R3/R9) and be connected directly to the 10k anode resistors. Cath. resistor should be around 650ohm.
As it is right now (referring to the schematics) the symmetry of the voltage swing is not so good. It needs higher B+. I am just saying this because the Maida reg. can really sound bad if it's not set up correctly with the rest of the circuit and that wouldn't show what Steve would want to find out: improvement or no improvement.

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Tue Nov 12, 2019 5:14 pm

Only just come to this reply from Wolfgang, because today, I’ve been having a play with choke-loading the driver stage, in order to clean up the driver swing. I couldn’t fit the chokes inside the box, so they’ve had to go on the outside, between the two 6H30pi. This valve cries out to be choke loaded, with its 840R nominal plate resistance. The fact that the chokes are outside doesn’t matter, as I’ve already decided for next year, to do a rebuild of this amp onto a new top plate and will find some space inside when I do said rebuild.

I now have 110V at the driver op point, with 6V of cathode bias, which works out at 12mA idle current according to the chart, which equates to a 500R cathode resistor, which is close enough for jazz, to what is actually on the driver board. So now I should get around 170V pk-pk of clean (ish) swing with that bias. OK, strictly speaking, I could do with about a hundred more Ohms on the driver cathode than I have, but it’ll do for now.

The two chokes are side-by-side in the same orientation, due to the lack of space, so one is wired so it bucks the other. As a result there is no increase in the noise level. The amp remains dead quiet. It also sounds bloody good. I can’t get any more power out of the output stage of course, but there is a bit more authority to the presentation. In terms of micro-dynamics, midrange and treble texture, it’s beautiful. The bottom end remains solid, tight with no chokey resonances or loss of extension. This amp is the gift that keeps on giving.

BTW the schematic is now obsolete. The present amp circuit bears little resemblance to what that schematic shows.
I’ve taken it down, in order to avoid further confusion. The hand drawn ones from early in the thread are still there as the context for them is still valid.
I’ll put up something at some point.
On every tree there sits a bird,
Singing a song ...of love...

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Wed Nov 13, 2019 5:33 pm

Tidied it all up this afternoon. The choke-loaded driver stage is a keeper.
53117726-C743-4147-8648-5B26CADB06C8.jpeg
It’s a long time since I choke-loaded a driver valve. Last time I did it was with a DRD amp, using 2A3s. I can’t even remember what valve I used as the 2A3 driver; probably something not really suitable for choke loading.
I had always known that I didn’t have enough HT for the input/driver stage given the transformers I had in the cupboard, hence my eventual choice of the 6H30Pi cascade over the 6SN7 Aikido setup.

I had considered choke-loading the driver at the planning stage, but talked myself out of it because this amp has lots of global feedback, to lower the output impedance enough to drive a speaker. I had read on the web that interstage transformers/chokes and global feedback do not mix, which is why they only tend to appear in SET amps that don’t use feedback.

Nevertheless, the choke-loaded driver stage works a treat and sounds great. I’m no engineer, but could the reason it works be that the amp is three stages and the choke load is in the second stage, so it is nested within the global feedback loop rather than having the choke sitting on top of the first valve, ie sat on the one being ‘fed back’ to?

I’ve changed my mind about rebuilding the OTL, as I don’t want to lose the sound somewhere and not be able to get it back. This means I now need to look for some kind of cover for the two anode chokes.
Last edited by Cressy Snr on Wed Nov 13, 2019 6:16 pm, edited 1 time in total.
On every tree there sits a bird,
Singing a song ...of love...

User avatar
Ray P
Social outcast
Posts: 3596
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Wed Nov 13, 2019 6:13 pm

There are various transformer covers, round and boxy, available from Chinese sellers on ebay.

User avatar
Ray P
Social outcast
Posts: 3596
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Wed Nov 13, 2019 8:29 pm

Wolfgang wrote:
Tue Nov 12, 2019 3:08 pm
I am just saying this because the Maida reg. can really sound bad if it's not set up correctly with the rest of the circuit and that wouldn't show what Steve would want to find out: improvement or no improvement.
Unfortunately, 150V is what I'm assembling one of the boards for so that's what I'll have available to lend Steve. I'm open to selling one of my maida reg boards but then that would jut add cost as all the through-hole parts would be needed for the required configuration.

User avatar
Ray P
Social outcast
Posts: 3596
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Wed Nov 13, 2019 8:30 pm

Interesting development Steve. You seem like a pig in the proverbial!

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Wed Nov 13, 2019 9:11 pm

Ray P wrote:
Wed Nov 13, 2019 8:30 pm
Interesting development Steve. You seem like a pig in the proverbial!
Yes, a lot fun work has been done on this amplifier in the last couple of months, starting with a visit with the amp, to Steve’s place. The three separate power supplies happened as a result of that visit. The choke-loaded driver stage is the icing on the cake. It’s all old school stuff on the front end, with valve rectified CLC power supplies and inductor load for the driver. Nick, Mark, Paul and JamesD bin there, done that, moved on. The concession to modernity with the little bit of silicon in the output stage cathode, keeps things reliable:)
On every tree there sits a bird,
Singing a song ...of love...

User avatar
Mike H
Amstrad Tower of Power
Posts: 18626
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

Re: Remember These?

Post by Mike H » Thu Nov 14, 2019 4:07 pm

Cressy Snr wrote:
Wed Nov 13, 2019 5:33 pm
Tidied it all up this afternoon. The choke-loaded driver stage is a keeper.

It’s a long time since I choke-loaded a driver valve.
Very nice. I want to do choke-loaded driver next, am planning another iteration of my 801A SET.
 
"Entrenched convictions cannot be changed by facts." – Dick Francis

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Thu Nov 14, 2019 4:39 pm

Mike H wrote:
Thu Nov 14, 2019 4:07 pm
Cressy Snr wrote:
Wed Nov 13, 2019 5:33 pm
Tidied it all up this afternoon. The choke-loaded driver stage is a keeper.

It’s a long time since I choke-loaded a driver valve.
Very nice. I want to do choke-loaded driver next, am planning another iteration of my 801A SET.
Can’t beat a bit of chokey action. :)

Speaking of chokes, I managed to find a chip serving basket in our local B&M for £1.49:
F2F79C5E-393D-4EA3-9865-BBF152C7D1E0.jpeg
The basket works as a cover and draws attention away from the two chokes.
Looks honest and industrial, and way better in the flesh than in the picture.
On every tree there sits a bird,
Singing a song ...of love...

Cressy Snr
Social Sevices have been notified
Posts: 8702
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Thu Nov 14, 2019 7:29 pm

Nah it looks fecking horrible.
I took it off. It looked like a pair of chokes with a chip basket over them. :lol:

It’s now been cut down in height, and turned through 90 deg.
Much better.
On every tree there sits a bird,
Singing a song ...of love...

Post Reply