Amp combo PreAmp out idea

[tubenit]

This idea is built on the Preamp & FX idea from this thread:

http://www.el34world.com/Forum/index.php?topic=15921.0

I decided to start a new thread to keep the ideas separate even though this borrows from the other one.

I got to thinking ............... " how can I have a low-Z preamp out line in a combo amp?"

So, this is what I came up with.  GlennJeff has already proven the line out idea works with a soundclip (which I thought sounded really good).

IF I am thinking thru this well,  it would work this way:

1)  In a setting where one could not use the amp combo and speaker and was required to run a line out into a PA system, one would
     dial the PPIMV down to zero killing the volume to the speaker.

2)  Then throw the DPDT which would engage the hi-Z/low-Z trannie.  This would also ground the signal into the LTPI.

What do you guys think?   Would this work OK?

With respect, Tubenit

[silverfox]

I don't know if this is along the lines of what is being discussed but Trace Elliot does a speaker emulation recording line out. See attached

[Glennjeff]

Just a thought tubenit,

Sometimes you might want to have both combo output and line output. Reducing the number of mic's onstage or recording a gig would be an example of when that may be handy. With a switch one can be flexible.

All the best.

[tubenit]

Silverfox,   Thanks for the schematic!

Sometimes you might want to have both combo output and line output

This is one way that could be done.

With respect,Tubenit

[jjasilli]

I'm thinking that the PPIMV makes the SW's unnecessary.  W/o the SW's the FX Send will always be active.  To kill speaker vol., turn down the PPIMV. 

Perhaps you may also want a Hi-Z FX Send, and also a Return.

[LooseChange]

Staying with the theme "Keep it simple"...
Use one switch. Always feed the line out but just use the switch to mute.
I also think the pot in the PI is unnecessary.

[jjasilli]

Yes, to mute the power amp, your option:  a mute SW per loosechange's post in your other thread; or no SW and use a PPIMV (actually the MV could be anywhere such as pre-PI, so long as it is after the FX Send; but I think you prefer PPIMV).

[terminalgs]

As for the output transformer (someone help me with the math):

You have two optional OTs: a 10K:150 and a 10K:600

So a 10K:600ohm  is a 4:1 transformer in terms of winding count.   If its to plug into a house PA or recording console, you'll want 'pro audio' line level, which means  between +4dBu and maybe +6dBu is a good target to aim for (I've seen some devices with a max. input of +25dBu, which is pretty huge!).   So,  Maybe aim for 4v p-p?  working backwards through the OT,   you want a 16v p-p signal on the primary side.

BTW: is the 10K:150 really the same 4:1 / 10K:600CT with the two secondaries strapped in parallel to make it a 150ohm? If so, it still 4:1 OT, and you'd still want 16v p-p, if ts really a 10K:150, its an 8:1 and you'd want 32v p-p on the primary. big difference.

Lets stick with a 4:1,  16v p-p is a low expectation that part of the ciruit,  If its 40v p-p  on the primary, then you'd get
10v on the sec, or about +12dBu, which might still be okay.. whats the p-p signal voltage in that part of the circuit?  

If you always play the amp at max volume, you can set the Lo-Z's p-p voltage based on some set two-resistor voltage divider coming off signal en route to the PI.  If you want to use the amp at low volume sometimes, and high volumes other times, you might want to make that voltage divider a pot so you can turn down or turn up the lo-Z's output.

Is this too simple?   replace a 1M grid-leak resistor with this 1M pot?  or replace that switch you have between the .22 and .022 on the PI with this?  



If too simple, there here are two others I have in my notebook (first is mostly from a Jensen whitepaper):



and



also,

I'd also switch that entire FX loop out from the first 12au7 section's grid to the second section's plate coupling cap out..,  If you aren't using the FX loop, you can take two triodes and 4 caps out of the signal's path,  and really Keep It Simple.

Also, the FX send-level pot is superfluous right?  In both cases of "12AY7" or "12AY7+5751" there is a level pot immediately before the FX buffer...

[tubenit]

Also, the FX send-level pot is superfluous right?

I have built 4 on board and two off board active FX loops.  The FX send, the FX return and the FX level all shape tone pretty significantly.
They may be superfluous regarding volume but not tonally, IMO.  

I was interested in the pot prior to the trannie also & had a 250ka pot there originally. On the other thread, jjasilli expressed concerns that it would be a bad idea?

Here's the thing ........... I am trying to recreate the D'Mars ODS  clean, OD and FX tone as closely as I can. So in light of that, I am inclined to build the thing in the DRAFT prototype that I have with the understanding that I can "correct" and change things as needed.

The current design is something of a compromise between what has been suggested, what GlennJeff has shown to work effectively and the original D'Mars ODS topology.  I think it's a reasonable risk to have it be the Prototype.

GlennJeff posted a very nice sounding sound clip with this idea behind it on the Tube PreAmp into Solid State thread. He was using something very close to this idea with success, IMO.

https://soundcloud.com/glennjeff/dmarscln2loz

Thanks for the input & I will keep track of it and use it as a guide if the design needs altering.

With respect, Tubenit

[jjasilli]

I was interested in the pot prior to the trannie also & had a 250ka pot there originally. On the other thread, jjasilli expressed concerns that it would be a bad idea?   Not terrible, but serving no purpose while needlessly causing more impedance mismatch from the 12au7 to the edcor primary.  (If you really must have a pot after the 12au7, then a pot after the edcor secondary is better IMHO.  Per the Rule of 10's such a pot's value should 600R X 10 = 6K.  So a 5K or 10K pot should work, I think.)

You have two optional OTs: a 10K:150 and a 10K:600  Either one should be OK; but I think the 10K:600 is better.  Per the other thread you probably want at least 200R on the edcor secondary to feed the SS power amp.  So, 150R starts off a tad low.  If the 12au7 circuit is down to 6R or 7R output impedance -- about a 1/3 mismatch to the 10K edcor primary -- that will bring the 600R secondary down to our floor of about 200R. Hence my preference.

[silverfox]

Regarding the TVT-9 schematic: I should have submitted this sooner but forgot all about it. The PreAmp schematic linked above, drives Opamps into a couple of MosFets in the power amp. The cleans are very much so and the distortion on the second channel is very clean also. As in not like a Marshall at all. Too sterile. That is why I ended up embarking on the journey that brought me to this forum.

Anyway, perhaps the circuit above is relevant to the topic of driving a ss power amp. Maybe here??

Silverfox.

[Glennjeff]

Don't know which thread I'm supposed to be using but here is the D'Mars OD channel with a Hi to Lo transformer hanging off the valve cathode followers cathode coupling cap.

There is no problems with level, I had to use a divide by 10 cable to stop it overloading a line input (-10dbU Line in I think)

https://soundcloud.com/glennjeff/dmarsod1

[DummyLoad]

you want the 10K:150 - 8.2:1

--pete

[jjasilli]

Pete, per the above posts, we have come to opposite conclusions.  What is your reasoning?