OTL power amps, power and price

Roger A. Modjeski · « **on:** 23 Oct 2011, 04:09 am »

I am happy to see the interest in OTL amps. As most of you know I designed the Counterpoint SA-4 which some say is the best amp they have ever heard. To my knowledge they made 50-100 pair, all on PC boards. Mine will be hard wired, simpler and more user friendly.

The big question is size. The power output of any OTL is limited by the maximum current one can safely draw from the output tubes. This is about 1.5 amps for the tubes I will be using. Peak power is that current squared times the load impedance. Your RMS power is half that. OTLs do not like RMS testing so that the RMS power should be rated rather low and one should be more concerned with the peak power.

Good speakers have become more efficient since I designed the SA-4 and many listeners are now happy with more definition and less SPL. Therefore we must re-evaluate out power needs.

Please vote.

bullwnkl999 · « **Reply #1 on:** 23 Oct 2011, 01:24 pm »

With my listening levels and sensitive speakers I rarely use more than 1 watt.

You asked on another string if anyone would want a pre/main separate switch and jacks?

Absolutely! With that feature and a low enough price I could get two and finally realize my dream of bi-amping with OTLs. Could have done it with two of Bruce Rozenblit's low power amps, but I'm not a big fan of his designs for several reasons.

rbwalt · « **Reply #2 on:** 23 Oct 2011, 02:41 pm »

OTL's:

sound open and uncomplicated with the best HF. No punch and extension in the bass and not as much layering & depth.

so no transformer no bass extension , no depth and layering to the music.

roger can these issues be addressed in a OTL?

rob.

Clio09 · « **Reply #3 on:** 23 Oct 2011, 04:01 pm »

Quote from: rbwalt on 23 Oct 2011, 02:41 pm

OTL's:

sound open and uncomplicated with the best HF. No punch and extension in the bass and not as much layering & depth.

so no transformer no bass extension , no depth and layering to the music.

roger can these issues be addressed in a OTL?

rob.

My OTL (zero feedback, 30 watts) has plenty of bass extension, depth, and layering. In fact I have compared it to my EM-7 12v and RM-10 MkII and it gives up nothing to these two amps in those areas, while providing some additional benefits, one of which is a bigger sound stage, most notable in depth.

My previous OTL (feedback, 15 watts) also exhibited similar qualities, although it didn't make the cut against the EM-7 12v or RM-10 MkII for other reasons. If I had to knock OTL's I'd say they could be quieter, but with my current OTL I'm really splitting hairs here because any noise I hear is with my ear to the driver, and it's just some 120Hz noise. I'm positive Roger will ensure his design is extremely quiet, but not sure why he would have to address the other issues.

roscoeiii · « **Reply #4 on:** 23 Oct 2011, 04:10 pm »

Yeah, bass extension has never been a problem with my OTL. In fact I went from an Pass Aleph 30 to my Atma-sphere S-30, in large part because of the better bass performance with my speakers. I have full range drivers, and they were overdamped by the Aleph 30, a problem I didn't have with most of the Nelson Pass First Watt designs (F1, F2, F3 were all great in terms of bass, especially the F1).

Roger, when you give these power numbers, I assume you are referring about OTLs into 8 ohms? (since power output goes up into higher ohm speakers, for those less familiar with OTL design).

FullRangeMan · « **Reply #5 on:** 23 Oct 2011, 06:13 pm »

Hi,
I need a monoblock passive/integrated(just one input for CD) OTL with two 6C33 for about 20W.
No PushPull, preference to SE Parallel.
The speaker is a FR crossoverless, OTL with popular tubes are useless to me, as I stock some 6C33.

roscoeiii · « **Reply #6 on:** 23 Oct 2011, 06:20 pm »

Also occurs to me that if folks are going to discuss their experiences with OTLs here, we should specify exactly what OTL you had experience with. Not all OTL amps are created equal. Some are more reliable than others, some have greater negative feedback, etc.

steve f · « **Reply #7 on:** 23 Oct 2011, 07:30 pm »

Mine is a Transcendent Sound. I bought the single ended OTL that puts out 1.5 wpc. Needs efficient speakers, but very full ranged and detailed sound. I have also tried their 40 wpc monoblocks which is a pp class ab amp design.

Steve

zybar · « **Reply #8 on:** 23 Oct 2011, 08:17 pm »

My OTL's are 140 watt Class A mono blocks. Given the relatively low sensitivity of my speakers (Vandy 5A's), lower power amps aren't the best choice.

George

Clio09 · « **Reply #9 on:** 23 Oct 2011, 08:19 pm »

I am also using an Atma-Sphere S-30 and for a while simultaneously had the Transcendent Sound T-16. The S-30 stayed in part because my Audiokinesis Jazz Modules were voiced with this amp and they prefer a higher output impedance from the amp. The S-30 (zero feedback configuration) is in the 7 ohm range while the T-16 was 1 ohm. Just an overall better match for my speakers which are 12 ohm maximum, 8 ohm minimum, and pretty much 9 - 10 ohm throughout most of the frequency range. In fact, I use the 12 ohm configuration on my EM-7 12v which sounded better than the the 3 ohm configuration with these speakers.

steve f · « **Reply #10 on:** 23 Oct 2011, 08:25 pm »

George,

How can your amp run in class A? The power line requirements would be industrial.

roscoeiii · « **Reply #11 on:** 23 Oct 2011, 08:32 pm »

Clio09,

I have many times wondered about the Transcendent OTLs. Can you say more about the differences that you heard between the 2 in your system?

BTW, for those of you with speakers that don't require much gain, I highly recommend the jumpers available for the 2nd pair of 6SN7s with the Atma-sphere S-30 (and I assume the Atma monos too, but not certain).

zybar · « **Reply #12 on:** 23 Oct 2011, 08:36 pm »

Quote from: steve f on 23 Oct 2011, 08:25 pm

George,

How can your amp run in class A? The power line requirements would be industrial.

Not quite at industrial levels, but they do use a lot of juice.

I run my whole system on a dedicated 20 amp line. Each amps pulls around 500 watts from the wall when turned on. So figure around half of the 20 amp line goes toward powering the amps.

George

jtwrace · « **Reply #13 on:** 23 Oct 2011, 08:40 pm »

Quote from: steve f on 23 Oct 2011, 08:25 pm

George,

How can your amp run in class A? The power line requirements would be industrial.

http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713

Roger A. Modjeski · « **Reply #14 on:** 23 Oct 2011, 08:48 pm »

Quote from: roscoeiii on 23 Oct 2011, 06:20 pm

Also occurs to me that if folks are going to discuss their experiences with OTLs here, we should specify exactly what OTL you had experience with. Not all OTL amps are created equal. Some are more reliable than others, some have greater negative feedback, etc.

To compare amps these are the relevant things:

1. power into what impedance
2. number and type of output tubes per channel
3. output impedance or damping
4. amount of feedback
5. retail price

steve f · « **Reply #15 on:** 23 Oct 2011, 09:00 pm »

Roger,

There seem to be three popular methods of OTL. Futterman, Circlotron, and Rozenblit. Could you please comment on them ( for educating us) and the popular tubes used? There probably are only about a dozen people in the country that understand OTL amps. Thank you.

steve

Roger A. Modjeski · « **Reply #16 on:** 23 Oct 2011, 09:01 pm »

Quote from: jtwrace on 23 Oct 2011, 08:40 pm

http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713

I spent some time on this thread a few days ago. I couldn't make much sense out of it. Please help us. Has Ralph spoken to this topic?

zybar · « **Reply #17 on:** 23 Oct 2011, 09:16 pm »

Quote from: jtwrace on 23 Oct 2011, 08:40 pm

http://www.audioasylum.com/cgi/vt.mpl?f=otl&m=33713

Seems like a pretty worthless thread to link to. There is no substance, just some troll making a comment.

George

zybar · « **Reply #18 on:** 23 Oct 2011, 09:19 pm »

Quote from: Roger A. Modjeski on 23 Oct 2011, 08:48 pm

To compare amps these are the relevant things:

1. power into what impedance
2. number and type of output tubes per channel
3. output impedance or damping
4. amount of feedback
5. retail price

Since you asked Roger...

Most of what you are asking for with my amps in on the Atma-Sphere website.

http://www.atma-sphere.com/Products/#MA-1

George

jtwrace · « **Reply #19 on:** 23 Oct 2011, 09:27 pm »

Quote from: zybar on 23 Oct 2011, 09:16 pm

Seems like a pretty worthless thread to link to. There is no substance, just some troll making a comment.

George

Really? I find Ralphs technical comments interesting from what I remember.

Poll

What level of OTL do you think you would buy

OTL power amps, power and price