[Jampot]

Thanks for that Roger.

Is there a 'minimum' line voltage required to get a good result from the EQ?

If I understand correctly, the usual line out voltage of a cd player or such is around 2v but when the kit is modded a favourite area for improvement is to remove (typically) underperforming op-amps and the result is to reduce the output to 1v or less. Will this still be sufficient?

I notice in his thread about suitable amps (for his Omega XRS speakers) Schotter already has his EQ - fast work

Jim

[Roger A. Modjeski]

good question. Being passive the EQ can only boost as much as is lost, which is about 7 db in this case.  One volt is adequate output if your power amp is very sensitive or you have a preamp. Schotter actually has too much gain in his system and the loss will be a benefit. For him to use the EQ we add an input selector in the same box.

If you have a system where the volume gets very loud at low settings the loss in the EQ becomes a benefit as you get to use the upper range of the volume control. Anytime you are stuck in the lower settings of your volume control you have too much gain in the system. Disagree if you like, but making the loudest level you require full up on the control has the benefit of finer adjustment, improved frequency response and the safety of someone turning your system up too high. Remember that a volume control is an attenuator.  If you only get to 1 or 2 out of ten you are throwing away 80% of the input signal only to amplify it again.

[schotter]

Will report back once that bad puppy is here.

[Jampot]

Schotter,

Anything to report?

Jim

[schotter]

Anything to report?

Yup, sorry for the silence, the puppy has been here for quite a while and we have been playing a bit...now, just to make clear where I am coming from: I have a set of ears and love a wide range of music but that's about it, I do not know what capacitors and resistors do other than probably resisting and such. In short, I am my own 'wife test' when it comes to audio.
First off, the EQ did make a huge difference in a pleasing way: Bigger lower mids and bass. Guitars have more body and some unwanted 'airiness' in voices does disappear. Johnny Cash is back to 'manly' and my 'AC/DC test' let me sit through half an album (to use some popular reference) even though I urgently had to get back to work. Again, when I am talking about 'bass' then I mean 'impression of bass', it could be the lowest of low mids. Fortunately Roger included an on/off switch to make the point so whenever I went back to 'flat', it sounded just very...flat. Except for some small chamber music and sparse indie pop. First round was won. But then I quickly started switching off my sub, due to bass boom in some of the material I listen to. I have an integrated so my sub will always get the same signal as my main speakers and with the beefier mid bass came a bigger bass impression overall. BTW, this regards only my CDP, my phono really hits a sweet spot with the EQ as I received it (Denon 103, Origin Live OL1/RB250 arm, Garrard 301, LeBong/small german manufacturer stepup and Lehmann Black Cube) and will be set this way. Roger suggested different sets of resistors and this way I could finetune. Which took me some time but then worked out quite well: A set of slightly different (one step up in numbers) resistors are giving me pretty much exactly what I want to hear from my CDP and now bass is just under the radar enough to let the sub do its duty.
Which means that I will now use different equalization for different sources.
And not get a new tube amp for now (although the EM7 idea does look tasty...) but I will probably test my CDP/speakers with a tube integrated as soon as I can.
The EQ is definitively worth trying if you have small single drivers and if you want to tilt the balance a bit.

cheers and thanks to Roger for his patience.

[Jampot]

Thanks for that Schotter - you must be pleased it worked out so well.

I'd better email Roger...

Jim

[rjones5296]

Hello
I built the Rosenblit OTL that was in Glass Audio years ago.
I actually built a set of 4 tube mono blocks using the 6J5 as the driver.
One thing I had problems with, was loosing output tubes, if the amp even slightly over loaded usually it would kill a tube.
The changes I made were to lower the B+ and B- for the output tubes from 140v to around +- 100v
This made the amplifiers reliable, not many problems after that.
Several years later I changed the input tube circuit to a +- 300v supply, using the same 12Ax7, this changed the character of the sound to a bit leaner, I say a more accurate sound versus tube warmth.
Since then I have used these amps nearly every day and still on the same set of tubes.
On my amplifiers I used a torroid transformer for B+ and B-, a seperate transformer for fillaments.
also another transformer for the high voltage B+ to the regulator for the input stages.
If I remember correctly there are 11 different voltages to power this amplifier.
If anyone would like to see photos of the completed amplifiers ill be glad to post.

bob

[Ericus Rex]

Sure!  I'd like to see them.  I'm in the process of reading Rosenblit's book right now.  Often wonder how his designs are.

[rjones5296]

Hello
Here are a few photos of my amplifiers.
I have the photos hosted at

http://www.pbase.com/rjones5296/otl_amp&page=all

bob

[markC]

Um... back on topic.
I like the format of your circle, Roger.
I believe the no nonsense approach and logic applied to audio electronics are the most beneficial for those wanting to learn.

[Ericus Rex]

Nice amp, Bob!  Very clean and professional looking.  Might be a fun project for me!  What the parts cost you, roughly?

[Roger A. Modjeski]

I looked over the schematic and it looks fine, just like a Futterman rearranged. I recall reading an article by Rosenblit in Glass Audio some 15 years ago. I do recall I was not in agreement with the article. Can anyone provide a link?

To the builder: I presume you keep the output offset balanced by fiddling with the bias pots. How much does it drift?

[rjones5296]

Hello
Ericus Rex the cost of both amplifiers was around 500-600, Alot of the components I sourced locally.
At the time we had a huge electronic surplus store. At the time I could get just about any small part for change on the dollar.
The transformers,chokes and chassis were new.

Roger A. Modjeski
I do adjust the offset ballance using the bias pots, the amps have a small Weston meter that to read voltage voltage drop over a two resistors one on the negative side and one in the positive side,  the resistor is bypassed by a switch when not setting bias.
The amps drift a bit, when warming up but settle down to a few mv when warmed up.
On the original Rozenblit circuit, I think he had the B+ b- voltages at 140, I have dropped this to something like +-110.
The higher voltage tortured the output tubes, i blew a couple sets. since that i have been on the same set of tubes for years.
I did check the offset the other day, and it was something like 5mv, I have not touched the bias in a year or two, so once set it is stable.
My friend Joe Tritchler re designed the input stage tube 1,  have a look and see what you think if you would.

I do drive this amp with the RM5, thanks, that is my favorite preamp. It has performed flawlessley for 10 years.

bob

[rjones5296]

Hello
About my OTL, I have been thinking about building another OTL similar to my first, except this time around I would double the output tubes, to 8 for each channel.
Im thinking this time on a 5u or so rack mount chassis, tubes mounted horizontally.

bob

[Roger A. Modjeski]

Consult the tube manual as to the alignment of the tube pins for horizontal mounting. The problem is grid sag and I wouldn't mount tubes horizontally unless I had a need to. They like standing up much better.

[acresm22]

good question. Being passive the EQ can only boost as much as is lost, which is about 7 db in this case.  One volt is adequate output if your power amp is very sensitive or you have a preamp. Schotter actually has too much gain in his system and the loss will be a benefit. For him to use the EQ we add an input selector in the same box.

If you have a system where the volume gets very loud at low settings the loss in the EQ becomes a benefit as you get to use the upper range of the volume control. Anytime you are stuck in the lower settings of your volume control you have too much gain in the system. Disagree if you like, but making the loudest level you require full up on the control has the benefit of finer adjustment, improved frequency response and the safety of someone turning your system up too high. Remember that a volume control is an attenuator.  If you only get to 1 or 2 out of ten you are throwing away 80% of the input signal only to amplify it again.

Roger, I have a MR 13em7 that I LOVE. Have played around with a variety of passive volume controls, but my current fave is a shunt-to-ground jobbie using 1k resistors and a 10k Noble pot. This pre only gives me the first half of the volume range and then it's too loud (Klipsch LaScalas are my speaks). But when I compare this with regular pot-in-a-box style passives (not yours specifically) at roughly the same volume level, the shunt-to-ground also seems to have more dynamics and punch. With the shunter set to 11 o'clock and the pot-o-box set to 2-3 o'clock (this give me roughly the same volume level from each), the shunter just sounds much more alive...almost like an active pre. Am I hearing what I think I am, or am I hearing something else? I've wondered if I'm "overdriving" the amp with the shunter, but that seems unlikely with a passive device.
Would love your thoughts on this. And let me say again, the 13em7 is a jewel!