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Old 15th January 2003, 12:20 AM   #11
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Great! Thanks a lot. That is exactly what I was hoping to hear. Also, you metioned the LL1660 and that's exactly what I'm doing! I have some 6h30's that I'm going to use as a long tailed pair to drive the LL1660, but I'll have to think of a good phase splitter to put in between the cascode and 6h30's. The LL1660 I have is the push-pull.

Thanks again everyone for your input so far!

- matt mitchell
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Old 15th January 2003, 01:01 AM   #12
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Quote:
Originally posted by mwmitchell
Great! Thanks a lot. That is exactly what I was hoping to hear. Also, you metioned the LL1660 and that's exactly what I'm doing! I have some 6h30's that I'm going to use as a long tailed pair to drive the LL1660, but I'll have to think of a good phase splitter to put in between the cascode and 6h30's. The LL1660 I have is the push-pull.
Matt,
Why do you need a phase splitter? Balanced in -> differential cascode (balanced out) -> diferential 6H30 loaded by LL1660 -> balanced line level out.

Here's a couple of tips.
- You probably won't need all the gain, so also consider using 12B4As in the line stage at about 120Va-k, 25-30mA/tube. Cheap available and very nice sounding. The 6h30 likes to be run at similar voltage but slightly lower current (20-25mA), so you can try both with the same PSU.
- Use a CCS in the tail to force balance in the stage, whatever tube you use.
- Use the LL1660 step down if possible 2.25+2.25:2+2 (1dB loss 12B4A or 6H30 depending on gain required) or 2.25+2.25:1 (this would be good with ECC99 - actual signal loss with 6H30)
- Following the last point, you could design it so that you could use a number of different line tubes and OPT configurations, by simply wiring in multiple sockets, and maybe a plug/switch arrangement for the OPT. This will allow you to adjust the gain for different mics to keep your headroom, and let you select the gruntiest driver for long cable runs. Some Pro mic cables are very capacitive, and irrespective of what the meter-readers say, several times the required drive current always sounds better.
- use at least an LC + shuntreg PSU. Any information you lose in a mic preamp is lost forever. All filaments should be DC and spotlessly clean.
- Powertrans in a seperate steel case. Don't use the mic leads to power the preamp chassis. Use a seperate military multipole, so no one ever confuses the two.
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Old 15th January 2003, 01:10 AM   #13
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Default PS.....

I presume you've had a look around http://www.vacuumstate.com/schematics.htm , specifically this? Well at least the first part of the cascode, before the RIAA network.
<img src=http://www.vacuumstate.com/images/RTP3C.gif>
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Old 15th January 2003, 02:18 AM   #14
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Thanks again Brett. Here is why I was talking about a phase splitter...

I don't have any really good outboard recording equipment right now. My recording system is really good though. But I only have a (!) Mackie for mic preamps. So, I was originally going to make a nice mic preamp, but then I realized I should take advantage of the fact that I have these great transformers (4 LL1660's!) and build not only a mic preamp, but an eq/compressor as well, all in the same circuit. I know that's not really a great idea (maybe) for a super nice mic preamp, but I'm hoping it'll work.

anyway, after thinking about how to make a balanced eq circuit, I realized it'd just so much easier to use the cascode, and only take signal from one side, then the eq circuit would be very much simplified. Does that make sense? As far as the compressor goes, I think that would have been fine in balanced mode.

Does that make sense? Is there a better way possibly? I mean I REALLY want fidelity, but I can't afford anymore transformers right now!

Thanks for ALL of the great tips. Seriously, that really helps me out! I have seen the circuit you posted. I bought his book (preamp cookbook) and it is great! So inspirational to me.

matt

p.s. Brett, would you mind sharing your mic preamp circuit?
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Old 15th January 2003, 04:52 PM   #15
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Default Wright and wrong...

Far be it for me to question the mighty Allen Wright, for he can string more valves in series than me..

But.....
Am I in error here, or is there a fundamental design problem with the RIAA network?

What impedance feeds the time constant networks?
Is it the anode resistor and valve combination?
Should not you feed from a constant impedance source?
How constant is the source?
IMO It is not constant. It will vary for a number of reasons, including valve ageing and supply voltage.

Oh dear, I can see I'm going to get into trouble here..

I'm interested in comments, please tell me where I'm wrong.

Cheers,
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Old 15th January 2003, 10:27 PM   #16
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Default RIAA

Hi,

Quote:
IMO It is not constant. It will vary for a number of reasons, including valve ageing and supply voltage.
Is it ever perfect?

You're correct in assuming it is not constant.
The CCS will try its' best to keep it constant though.

Personally I don't like the way the RIAA network is done.I'd rather see an individual network for each input of the preamp.

Quote:
I'm interested in comments, please tell me where I'm wrong.
Spill the beans,John.Tell us more.

Cheers,
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Old 15th January 2003, 10:37 PM   #17
dhaen is offline dhaen  Europe
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Default RIAA

Sorry to hijack the thread....

Maybe this is the way it's normally done with valves?

Going back to basics, surely the RIAA (if of the passive shunt kind), would be best fed form a valve "op-amp" with low output impedance via a precision resistor.

My feelings anyway...

Beans spilled.
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Old 15th January 2003, 10:44 PM   #18
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Default HIJACK II

Hi,

Quote:
Maybe this is the way it's normally done with valves?
Hmmm...this is not your every valve preamp anyway...but IMO the best way is indeed to have a well defined low output impedance to drive the RIAA network.

The best is the LCR network at 600 Ohm in and out driven from either a buffer stage or a IS.

Going balanced is going to cost me though...

Why do you think I paid Benny G. a visit last Friday?

Cheers,
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Old 16th January 2003, 12:51 PM   #19
dhaen is offline dhaen  Europe
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So what are you doing Frank?
Is it a new project?
Maybe it deserves a thread, so we can all "tear it to shreads"

Cheers,
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Old 16th January 2003, 01:51 PM   #20
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Default Is it a new project?

Hi,

Not really new...something I want to try out together with some friends for too long already.

Looking at Benny's site brought the idea back up again.
It's going to be able to take any MC on the planet using valves all the way and perhaps a stepdown xformer at the output.
It's going to have an an absolute phase switch and if feasable an S&B VCT as well,if not economical a constant impedance L-type attenuator will have to do.

The PSU is all valve regulated on a per valve/stage basis...the rest of it I'll need to figure out later.

When it shapes up I may give you all a chance to shoot it down.

Cheers,
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