Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: EL34 on August 19, 2015, 09:12:22 am
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Over the years I have used my Stereo Tube preamp to feed a few different devices
http://el34world.com/projects/StereoPreAmp1.htm (http://el34world.com/projects/StereoPreAmp1.htm)
In the past I have always plugged my guitar into the input and fed the output to a power amp or to a computer sound card
Like this - Guitar > stereo tube preamp > Tube power amp
or this - Guitar > stereo tube preamp > Computer sound card line level in
Now I am using it between my Kemper profiling amp and a mixer to warm up the sound and get a bit of a boost
Guitar > Kemper profiling amp stereo lines out > stereo tube preamp > 8 channel mixer stereo inputs > power amp
The kemper is putting out a stereo line level signal
The 8 channel mixer is expecting a stereo line level signal
So now I want to optimize the inputs and outputs on my tube preamp for line level signals in and out.
I am thinking I should change the output pots to 10ka or 10kl for the mixer inputs?
What about the input jacks?
They currently have your standard 1m resistors to ground for guitar level inputs?
Notice the output stage is a cathode follower through a standard Marshall type tone stack
Any suggestions on other areas that may need changing?
Check out my schematic below and the red areas
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Guitar > Kemper profiling amp stereo lines out > stereo tube preamp > 8 channel mixer stereo inputs > power amp
The kemper is putting out a stereo line level signal
The 8 channel mixer is expecting a stereo line level signal
input impedance of your preamp: depends on the output impedance of the kemper. they have lots of outputs, monitor out, unbalanced, balanced, DI out, etc.. I'd look for the output that most closely resembles an instrument output,. It might be call "high impedance output", something in the 5K-15K range. maybe a 1/4" unbalanced in the 0dBv to +4dBv range? That is, if you want to maintain the ability to still plugin a guitar. Avoid low impedance outputs on the kemper, 30ohm to 1K ohm.
I think swapping those 1M's for 10K pots will change the way the T/S sounds drastically. Those caps (500pf,.022) have low-frequency roll off characteristics based on the the resistance to ground of the tone pots in parallel to the volume pot. I'd keep the overall resistance around 1M, and make it a resistor on top of a pot... i.e. a 470K R + 500K pot, or a 1M R + a 100K pot.
If you are finding that the preamp just has too much over all gain (after all, you can drive a power amp section with it...) I would look at adding voltage dividers at both volume controls. at the 1st control, maybe a 470K + 500K pot instead of the 1M pot.
If you have the real estate for two more output 1/4" jacks, and you want to maintain the ability to plug-in to a power amp directly, you could keep the 1M pot as is, and put a small pot underneath it with center wiper going to a low-level output, like a 100K pot. With those cliff jacks that have a switch on the "ground sleeve" of a TS, you could do some clever wiring to take the small pot out of the circuit when not in use.., so the 1M pot looks exactly as it does today if you want to drive a power amp.
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Basically, ditto to terminlags. I.e., it would be optimal to have impedance matching at the inputs and outputs of all the devices. But it might be good enough now, unless there are any actual issues:
* is the preamp happy with signal from the Kemper? (Is the preamp input getting swamped? Probably not.)
* is the Mixer happy with output from the Preamp?
* Is the Power Amp stereo?
* why the Mixer?
Terminalags touches on another point. The preamp, though doable, may be overkill. E.g. a tube buffer (1:1 gain with any needed impedance matching) may be sufficient.
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Thanks guys
I'll have to look at the Kemper outputs when I get to the shop, I am at home now
Why the 8 channel Mixer?
Because I mix different sources and feed them to a stereo power amp
Sources:
Computer motherboard sound card out for playing MP3's and computer sounds
Recording sound card out so I can play and hear mixes
Kemper guitar sounds
Channel 1 is the Microphone channel on the mixer
Other misc sources
I may just build another pre amp and leave this one alone
I really don't need a tone control section for what I am doing
I am only looking for a bit of boost coming out of the Kemper and some tube amp warmth
That's the mixer in the top rack spot
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I think the issue with the stereo preamp is that it has too much gain. You have 3 cascaded 12AX7 triode gain stages. I would add a switch to bypass V1-B/V3B. You might also consider replacing V1/V3 with 5751's or 12AY7's to lower the gain even more.
Another idea is making the inputs switchable to V2-A/V4-A. If the Kemper is providing volume control, you wouldn't need one on the preamp.
You could even put a pair of normalled jacks on the rear panel like send/return between V1-B V2A / V3-B V4-B.
You would just use the "return" to bypass the front end.
Jon
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> output stage is a cathode follower through a standard Marshall type tone stack
*And* a 1Meg Output pot.
Since mixer inputs today may be 10K, they will suck the life out of a 100K-1Meg output network.
The "simple" answer is a buffer after that Output pot.
This may not be simple on an already-built preamp. You don't have the low voltages for a chip, and you may not have space to add cathode-followers.
And yeah, that is a LOT of gain, so you *need* the Output pot.
At the inputs, if your line-level source is hot and not adjustable, you may want input pots.
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This may not be simple on an already-built preamp. You don't have the low voltages for a chip, and you may not have space to add cathode-followers.
Not enough low voltages for a chip op-amp, but how about a mosfet follower?
Jon
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I think it would be simpler to start with something new and specifically designed for how I am using it
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Doug,
Per the Kemper Manual pp. 21 - 22, there are 2 outputs: "Master Out" > mixer; "Monitor Out" > guitar amp/cab.
Hence, it appears that the Kemper's Monitor Out is already designed to mate with your tube preamp's input.
A twist: the Kemper Manual suggests that if you select Monitor Out > guitar amp/cab, "cabinet simulation" should be switched OFF in the Kemper. But, if your Power Amp is feeding full frequency PA speakers, you may need to keep the Kemper's "cabinet simulation" ON. That way you can preserve the 4kHz roll-off, etc., typical to guitar speakers.
Another possible option: maybe put a tube preamp or buffer Before the Kemper???
OTOH, I've had happy tonal results simply using a tube power amp, with no other tube devices earlier in the chain, just SS. But that's for stage use. Maybe recording is more demanding.
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I am using the main outputs left and right because I need stereo
I feed those into the stereo preamp and then into the stereo mixer channel
Monitor out goes to the input of my recording sound card
It's only a mono output and is better for the recording process
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Some more info on line level signals > guitar (pre)amp.
http://www.soundonsound.com/sos/jun04/articles/qa0604-3.htm (http://www.soundonsound.com/sos/jun04/articles/qa0604-3.htm)
http://forum.cockos.com/archive/index.php/t-19343.html (http://forum.cockos.com/archive/index.php/t-19343.html)
Bottom line, try it & see how it sounds.
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I have been using it as described below for a couple months
Guitar > Kemper profiling amp stereo lines out > stereo tube preamp > 8 channel mixer stereo inputs > stereo power amp
I am only asking about it because I would like to match the input and outputs better to the gear it is hooked up to
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Ok. Focusing on the Kemper > tube preamp connection, checkout this Jensen input tranny (1 per channel), to match a line level signal to a vacuum tube input stage: JT-11P4-1
http://www.jensen-transformers.com/transformers/line-input/ (http://www.jensen-transformers.com/transformers/line-input/)
Schematics for the JT-11P4-1 are on its pdf spec page.
Also, Jensen has a schematics page with circuits that address the issues on this thread:
http://www.jensen-transformers.com/schematics/ (http://www.jensen-transformers.com/schematics/)
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Thanks
The tranny prices scared me on that site
I can't view the schematics, not a member
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MOSFET buffer* to line level OT. keep the hi-z outputs as well. add jack switching so that when you plug in the hi-z jacks you disconnect the buffer input by grounding it.
* MOSFET buffer = source follower.
--pete