My DI Box

[analag]

My latest Direct Injection box.

It sounds like tube but more extended at both ends of the audio spectrum.


Analag

 

[Samuel Groner]

Thanks for sharing! Looks interesting and I have a few Qs for you:

Why paralleled input devices? Looks like overkill for 2k source impedance with these transistors and is for sure for anything above 5k. But in turn you'll get higher nonlinear capacity (OK, is cascoded).

Oh, and why not reference the cascode to the R4/R5/M2 node?

I'll save you: it sounds better this way, right? :wink:

Samuel

 

[analag]

I paralleled the FETs for safety. At full swing they conduct almost 10 ma so why not double them (to be safe)and reduce noise at the same time. I always drop a little resistance between the sources and the CCS to balance things out, if the parts were ideal like in SPICE then I would feel that I could drop these resistors out of the circuit. And last but not least...the sound.

Analag

 

[Samuel Groner]

Oh, that makes sense. I still wonder why you don't reference the cascode to the source resistors - this would greatly reduce dissipation, no?

BTW, are the currents in the two legs equal? R2=R3 does imply otherwise.

Samuel

 

[analag]

Referencing the cascode to the source resistors introduces nonlinearity when the input is driven hard. R2 R3 are theoretically balanced.

 

[Samuel Groner]

Can you explain? Never heard before...
Samuel

 

[analag]

Notice how Borbely implemented his hybrid cascode
http://www.borbelyaudio.com/eb1195221.asp
I'm using the same concept, but configured as a differential pair. The schematic is really a tube hybrid opamp driving a transformer. It's very simple and great sounding.

 

[Learner]

With the amount of experience and expertise in building amps such as Mr.Borberly, it would be very enlightening to know why Mr. Borbely does not utilize regulation in his HV PSU?

Is regulation of HV PSU in tube amps a waste of time and effort, that has little benefit to be gained???

 

[analag]

You mean regulating the B+ voltage for a power amp....that would be a waste of time an money, because the regulator would have to be able to pass at least twice the wattage to the amp it is supplying.
A 50 watt amp with a 100 watt regulator.... nope I wouldn't.

 

[Learner]

[quote author="analag"]You mean regulating the B+ voltage for a power amp....that would be a waste of time an money, because the regulator would have to be able to pass at least twice the wattage to the amp it is supplying.
A 50 watt amp with a 100 watt regulator.... nope I wouldn't.[/quote]

Right, I see.

I can't seem to find any other HV PSU on the site for tube pres, wonder what he would do in those cases.

Have to dig into HV PSU building thoroughly when I get time again.... sigh....

Got another month and half to finish my ucontroller analysis and design project and its on tubes!!! :grin:

 

[Guest]

I agree that regulation of the B+ supply is not necessary, but I don't think output wattage correlates to supply wattage.

The supply posted above has a pretty stiff B+ pi filter so ripple is minimized. Since it is a preamp, current demand is relatively stable so sag is not a real concern in this application. Regulation here, I believe is an unnecessary complication.

One other aspect to concider, B+ sag, in many tube amplifier applications, adds a bit of "flavor" to the audio. This is particularly noticable in power amp applications where the supply is working harder.

 

[analag]

That is a PSU for a 6C33C Single Ended amp....that should be a class A pwr amp if I'm correct.
A PWR Amp with a sagging B+ is an effect box. Right?

 

[Guest]

You are right. I didn't read it well at all.

There are a lot of factors determining power supply sag, but if the supply sags, it certainly affects the output. I wouldn't go so far as to classify it an "effect box" though unless it was an extreme case.

But we argeed on the need for regulation being unnecessary in this application. Are you now saying it is?

As far as wattage of the power supply is concerned, how does it correlate when the supply provides 210VDC at say, 250mA and I am driving an output transformer with an 8ohm load? I don't understand why the supply must provide twice the output wattage.

 

[analag]

During short transient peaks the amp draws much more than it is rated for from the PSU. The PSU must cleanly deliver what is required in order not to affect the sound of the amp. Designing a PSU for a power amp is quite different from designing one for a preamp.

 

[Guest]

We agree again on the differences in design requirements between power amp and preamp supply. In general terms, preamp supplies must be cleaner, maybe a bit stiffer and have much less current demand than power amps.

I don't really understand your comment about the amp drawing much more than it is rated for on transient peaks. Are you speaking in general terms or does this specificly apply to your DI?

 

[Kev]

[quote author="analag"]Designing a PSU for a power amp is quite different from designing one for a preamp.[/quote]

yep
and a power supply in a guitar amp
supply sag is favourable for a guitar amp

has anyone tried this for a mic-pre ?
might make for a very coloured and compressed tone

 

[adrianh]

Has anyone thought of HV pass regulators
"sound" kind of bad.
Plain old RC filtering "sounds" good?
With Choke input sounding the best.

Now shunt regulation applied correctly
can sound better than RC filtering.

I do not stop to analyze why. I just
build the gear. Theory and opinions sometimes do
not translate to good sonics. But when you hear good
sonics you know it. So build listen tweek.
Your ears will not lead you astray. Bad designs
that measure great will lead you down the sonic trashcan.

 

[analag]

Are you speaking in general terms or does this specificly apply to your DI?

In general terms. This does not apply to preamps.

 

[rafafredd]

what do you guys think about this one?

 

[analag]

Looks good, but I personally like a MOSFET pass device. A voltage controlled amplifier (source follower).