Redd 47 Pre with Drive+EQ DIY

[alexc]

Still banging away on the pentode .. 

It has a gain varying from around +32dB in full pentode mode, down to around +24dB in full triode mode.

Now that's a lot to throw in the path, especially when you have some pots'n'switches hanging off the plate and screen - hiZ = lotsa noise!

After trying a few things, I ended up using a pair of 2-core shielded + foil  as hookup for the controls.
I have the shield+foil grounded at both ends, in this case.

Now that's made a very big positive difference in the noise levels related to the control wiring.
Something like a 15dB improvement in headline noise, mostly at hum freqs but with some hiss reduction too.

With the pentode stage switched out, I now have my baseline noisefloor of around -82dBu or so.
With the pentode in +32dB I have a noisefloor of around -72dBu

So that's pretty good and should improve another dB or two when I have the controls mounted on the alu face plate rather than lolling about on the table

So - when dealing with hiZ, high gain -> shield + foil grounded is best!

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That's a big milestone because I've always had big issues at the pentode gain stage with noise.
Of course, it's quite microphonic as one would expect! I may try some tube dampening rings.

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The final checks are with pentode in, Redd 47 at unity gain with -10dBu line input

+32dB gain -> 'Drive' pot -> AU7 and line amp -> T-pad

with the T-pad at min attenuation and the 'Drive' set for unity gain at output

I get my nominal baseline noisefloor of -81dBu

Same thing but with the t-pad turned way down for 'strong atten' and the 'Drive' way up, even better at -82dBu.

So that's a wrap.   

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Now I have a working 'max pentode gain' scenario, I can go forward and figure what fixed atten to put after the pentode.

THEN, re-connect the EQ circuit and that's the hard part done. 

 

[alexc]

Before adding the EQ back in circuit, I put in a 'eq bypass' switch - it has a 12dB pad in the bypass position to more or less match a typical eq stack loss.

I also figured about 20dB is the max gain I want from the pentode, so I put a resistive divider on the plate output.

Once I put the eq back in circuit, I get a 'worst case' noise floor with flat eq of -74dBu with an output set for +4dBu. (which is close to the max my Motu test rig can handle).

That represents around +14dB overall gain xlr-to-xlr, not including the +12dB EQ makeup applied.

I think I can tweak that by maybe 3dB more once I button up the unruly front panel wiring.

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That's about as good as I can see myself getting with all the undisciplined wiring and such, and is actually pretty good.

It remains more or less the same regardless of the setting of the pre-line-amp 'Drive' pot setting and post-line-amp 'Level' t-pad.

The Orange86 module (redd47) feedback-attenuator 'Input' control does not affect the noisefloor at all, but does strongly influence the THD amount and makeup.

There is obviously a 'sweet spot' with the 'Input' and 'Drive'/'Level' controls for output level, noisefloor and THD. 

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This unit will drive my monster limiter, the pmSix61 which is quite happy with high input levels and has a very low self-noise.

So I expect that with an average output at around +4dBu, I should wind up with a noise floor of -77dBu for this unit which is pretty respectable and with the limiter after it, should remain around that level.

More or less.

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So - that's the noisey bits done.

Next to finalise the line amp 'Feedback' network - I may well retweak the gain staging once I see the effect of this control.

Then back to the Orange86 module to add the 'Squish' pot and tap into the LPF to extend into my 'HiCut' rotary.

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Next stop, I'll also have the front panel in place - I need that to 'get real' for the final lap.

I'm pretty sure this guy will be worth the effort in the end 

Cheers

 

[alexc]

Still haven't done the front panel 

But I *have* been playing thru this guy for a while now at the bench.

I truly think this is the best sounding preamp for the egituar I've built to date. It really does sound the closest to a tube amp set to be basically clean but with a little crunch in the bottom end and mids.

At the stronger crunch end, I probably could use a little bit more filtering of the top end to get rid of a touch of aharmonic sounding distortion in the 2K-5K region. But it's not too bad even at higher crunch.

Not a hint of the usual 'hissy-fizzy' business that overdriving grids gets usually gets you - classic 'boogie' preamp territory. This is way more like a marshall - visceral mids and growly bottom ends, with the tops mostly pure.

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I'm in 2 minds - either to button it up as is, or to go for a better EQ, like using Ian 'Ruffrecords' EQ pcbs to get more control.

It's sounding so good I think it deserves the 'ruffrecord' eq treatment to really tune things up.

I attribute the sound to  :

i) pentode first stage
ii) 4 stages of edcor and like traffos
iii) split PI 
iv) strong pp output stage with t-pad after traffo

The eq+pentode doesn't do all that much - the thd is all in the front end, the 'grunt' in the back end.

I will say the 'lift' control is cool and so is the morphing 'pentode/triode' mode.

The 'shift' not so useful - prolly ditch it.
The 'feedback' was a bust - didn;t do anything useful - gone.
So far the hi-cut filter not usesful either - gone.

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The noise performance is great - with strong levels being around 12db on EQ makeup and 12dB overall gain, I'm seeing around -76dBu noise floor 20-20K 'spl style' measurement.

That's on -84dBu 'loopback', so 8dB worsening - which is not bad at all.
Miles below the -60dBu or so where a gate starts becoming a necessity for good silence.

Due to  the excellent 4-stage HV pi-filter/regulator on the 'Orange86' (Redd47) module powering eveything, the additional pi-filter de-coupling of each stage's HV and also the 40KuF (on 2.5A) heater pi-filter.

As well as foil+shielded front panel wiring.

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So - to splurge some more cash and time. Or not - that is the question!

If I followed my heart, I'd put in Ian's Helios style eq.

That's what it deserves. If the noise performance wasn't up to scratch (which I fully expected), I'd not bother.

But it is. And Ian's Helios Eq would make it a truly unique box in my collection.

Oh well - there's worse things in life to weigh up 

 

[alexc]

Back again on this project after a long absence ....

I just finished this one. Here's the final inside pic.

 

[alexc]

And here's the front all done ...

 

[alexc]

It is performing really well - very quiet despite the 'prototype' level build values 

And it has quite a lot of places to dial in/out gain. 

Doesn't really do crazy over the top stuff but many shades of harmonic distortion. It's a very classy sound.

My absolute best yet - getting smooth crunch is best done imho with gentle gain staging across many, MANY tubes, transformers and filters

The EQ is actually quite tame but does a lot of unexpected things - which can be fun.

Frequency response is all there - no real issues to speak of.

So - perhaps a little more time to tune the EQ but prolly this guy goes into my music room today!

I'll be using it with my pMSix60 Limiter with a cheap SDC tube mic as well as others.
AND for DI the guitar/bass. I have a Bass VI which I plan to use with it - that will really shine there I think.

bfn
AlexC