R*E*D*D*ish mic preamp

[Rossi]

Thanks a lot, Winston :thumb: You eased my mind inedeed! :guinness: :guinness: :guinness:

I put a old (used) Valvo EF86 into channel A. With the stock tubes ChannelA was a bit noisier than Channel B. The Valvo was considerably lower noise, Channel A is now better than Channel B. I got another new or almost new Valvo that measured very well, but the noise on that one was higher. Seems like you have to go through a number of them to find one that's really low noise. I may be a bit pickier about noise than most people, though. The noise floor is not really a problem. As I said, the sound is really nice, no buzz. So I guess I'm fine. :grin:

The 206V HT voltage was measured at the PSU section. There is a test point or jumper that says 250V. Gotta take a look at the input transformer. The unit I have is indeed one of the later ones with the DI jack on the front pannel. I bought it early this year when the German distributor sold its remaining stock. BTW: I talked to a guy from the ex-distributor team at the Musikmesse, and there was a bit of regret given the popularity of the new models.

One question about the output impedance switch - are 600/10K the actual output impedances or are those the minimum load impedances? The manual is a bit unclear about that point.

Have a nice weekend!

 

[Winston OBoogie]

[quote author="Val_r"]
I love the sound of the 6n1-P, and it's a military replica of the E88CC
[/quote]

I haven't done harmonic analysis on a 6N1-P but I do think they sound quite similar to an E88CC when operated at the same points. To my ears there is an impression of detail which on an E88CC I've put down to a higher than average 3rd harmonic. Again, and as with the regulated H.T., it's part of the sound of a REDD.47. It isn't like the original EMI design engineer wasn't aware of it either as there are notes pertaining to it. Besides providing an easier load for the EF86, that local feedback loop around the E88CC is there to bring the valve's distortion down and more in line with that of the front end EF86.

I may be biased here but I have yet to find a detail of a design from this period where, in my very humble opinion, EMI made a 'mistake'. Granted, we could do things differently nowadays but bear in mind that most of the valve equipment was designed, or 're-designed & re-built' as in the cases of the RS.124 & the Fairchild 660, by someone who had 'understudied' Alan D. Blumlein. The same people who designed and built this equipment for recording sound were also designing and building Military computers and radar systems, coming up with T.V. operating protocols...I could imagine that working in R&D for EMI from the mid 1930's into the 1960's was maybe quite similar to being on the payroll at Bell Labs. in the U.S.

 

[Winston OBoogie]

[quote author="Rossi"]...I guess I'm fine. :grin: [/quote]

Good.

[quote author="Rossi"]...are 600/10K the actual output impedances?
[/quote]

No. The actual output impedance is lower although not as low as some amps because it's zero feedback circuit. The output stage uses a honkin' great transformer with the DC shoved up the primary for the // anodes. All the switch does is terminate or leave open the trannie's secondary. What you hear is a different tone due to the reflected load on the anodes changing. The 10K position is just a typical load that the pre will see when operating into a modern bridging IP.

Have a good weekend yourself, signing off for now...
Ciao.

 

[CJ]

pentodes require more sorting, for some reason.
maybe becuase you have 4 gaps between elements instead of two.

 

[Winston OBoogie]

[quote author="Val_r"]
Changing OT from 8.15:1 to 4:1 will probably help a bit, what do you think?
[/quote]

Some 6dB more gain but possibly too low a load for the // anodes. With no load on the secondary it might be OK but anything approaching 600R will lead you back down the path of excessive distortion from the 6N1P.

If this isn't a clone and you want more gain then a follower before the O.P.T. would allow for a lower turns ratio.

 

[analag]

If I were to stick sandstate in this circuit I would probably start out with something like this. (For demonstration purpose only)

analag

 

[analag]

If or when I get around to building the so called Rolls Royce of tube mic pre (V76) instead of using those frequency reactive inductor plate load, I might try the sandstate CCS load for a flatter and cheaper alternative.

analag

 

[Marik]

[quote author="analag"]If or when I get around to building the so called Rolls Royce of tube mic pre (V76) instead of using those frequency reactive inductor plate load, I might try the sandstate CCS load for a flatter and cheaper alternative.

analag[/quote]

As I wrote earlier, I tried it for the same reason--to save some $$$. While it sounded nice, I had to drop the idea due to the higher noise. I used a deplition mode MOSFETs.
Please report if you have a better luck with your arrangement.

 

[Winston OBoogie]

[quote author="analag"]If or when I get around to building the so called Rolls Royce of tube mic pre (V76) instead of using those frequency reactive inductor plate load, I might try the sandstate CCS load for a flatter and cheaper alternative.
[/quote]

Won't that mean it's no longer an erm... you know?...V76!
Just kidding :grin: go for it.

My experience is similar to Marik's in regard to CCS loads for valves to which I'd add that I also prefered the tone from a nice choke.

Got some experience with the V76 amp myself (I did the design work and PCB layout for the Mercury Recording V76m - seems like all I ever do is bloody mic pres!) and it's not too hard to get decent chokes wound. I'll add that a 3 chamber bobbin seemed unnecessary in my testing, a two chamber working very well. You have other issues that need addressing in order to coax the V76 circuit into behaving herself.

Ciao.

 

[analag]

[quote author="Marik"]As I wrote earlier, I tried it for the same reason--to save some $$$. While it sounded nice, I had to drop the idea due to the higher noise. I used a deplition mode MOSFETs.
Please report if you have a better luck with your arrangement.[/quote]
I ran the simulation with DN2540 depletion mode MOSFETs and it sucked, but showed great results with the arrangement shown above. The BJT configuration has a much stiffer current control, which is what is needed I think.
[quote author="Winston O'Boogie"]My experience is similar to Marik's in regard to CCS loads for valves to which I'd add that I also prefered the tone from a nice choke.

Got some experience with the V76 amp myself (I did the design work and PCB layout for the Mercury Recording V76m - seems like all I ever do is bloody mic pres!) and it's not too hard to get decent chokes wound. I'll add that a 3 chamber bobbin seemed unnecessary in my testing, a two chamber working very well. You have other issues that need addressing in order to coax the V76 circuit into behaving herself. [/quote]

The tone will probably be different due to the flat response of the CCS I suspect. I've played around with this kind of thing quite a bit with very good results, lower noise, faster sound, tighter punch.
I will try it one day, but who knows.

analag

 

[Winston OBoogie]

[quote author="analag"]
The tone will probably be different due to the flat response of the CCS I suspect...
[/quote]

Yep, that was my impression.
After evaluations, I came away with the feeling that the better of these CCS arrangements for anode loading were Gary Pimm's circuits. I pretty much copied them as presented on his site:

http://home.pacifier.com/~gpimm/


If cornered, I generally panic somewhat and pick an R, a choke or a transformer for the immediate load on the anodes though.
However, I'd be quite happy to throw good money away and use a CCS for sourcing to a reg. in the H.T. supply line.

[quote author="analag"]I will try it one day, but who knows.[/quote]
Multitudinous are the possibilities aren't they?


It just occured to me, Val's thread is getting a bit hijacked. Sorry Val

 

[Rossi]

[quote author="Winston O'Boogie"]
Got some experience with the V76 amp myself (I did the design work and PCB layout for the Mercury Recording V76m [/quote]

Seems like it's hard to buy a decent mic pre these days that has not been "Boogied" in some way. :grin:

Checked the IP transformers in my MP-2 - they're the standard Jensens (JT-11K8-APC). So they can't be responsible for the extra gain. Strange.

 

[SSLtech]

[quote author="Winston O'Boogie"]It just occured to me, Val's thread is getting a bit hijacked. Sorry Val [/quote]
I can't speak for Val, but I'm just grooving on readin' yo' rap, dude! :wink:

Keef

 

[CJ]

76 has two chokes....

distortion should be included in a choke vs r vs semi's.

 

[Val_r]

[quote author="Winston O'Boogie"]It just occured to me, Val's thread is getting a bit hijacked. Sorry Val[/quote]

:wink:

An updated diagram, with a mod that would be interesting to build:
inductive approach on the anode rail, keeping the ccs design for the cathode rail.
As soon as I get my hands on a JJ ECC99, will post the characteristic curves and spectral analysis, for those interested.
Total load that the entire ECC99 should see is 20K, which in the chosen working point, should have an extreme linearity.

Any comment/suggestion/opinion more than welcome!

Respect,

Val

 

[Marik]

Any particular reason you are using choke together with CCS? :shock:

Also, the ECC99 is quite a powerful driver. Why do you need it paralleled?

 

[Gus]

It kind of looks like a cut and paste design to me. CC with a pentode?????? CC with the choke????? Don't make any sense to me

 

[Val_r]

[quote author="Marik"]Any particular reason you are using choke together with CCS? :shock: [/quote]

I would like to experiment it.

 

[Winston OBoogie]

[quote author="Val_r"]
Any comment/suggestion/opinion more than welcome!
[/quote]

Just a thought based on my own doodlings:
This biasing arrangement may not provide the most benign overload recovery characteristics for a mic amp. I'd rather have a bit of fuzz than a SPLAT!

Might not be an issue for ya but thought I'd mention it.

 

[clintrubber]

Hadn't seen this way of doing the CCS before. I mean, with that Zener added - no noise problems ?

What are the actual benefits of this current source & folding with the Zener vs a 'normal' current sink ? Anything else than the headroom & fixing of the cathode-potential ?

Regards,

Peter