Followers and Quickie

[Paul Birkeland]

I just got an email from Mike Rothacher telling me that the Avid sink in the article( the one I used) is not large enough for the 193V version. PB, from your comments this morning I can tell you are fairly familiar with this design. Any suggestion for a larger heat sink?

Put a fan on it!

Also, do the suggested LED changes to the Crack make it less suitable for headphone use?

The amp will certainly sound different, but there should be no negative impact on headphone performance.

[Deke609]

Apologies, Jamie, for butting in. I'm trying to follow along and learn something

@PB: So swapping in larger voltage drop LEDs increases grid bias, limiting current through the driver?

many thanks, Derek

[Paul Birkeland]

@PB: So swapping in larger voltage drop LEDs increases grid bias, limiting current through the driver?

The first stage plates are loaded with constant current sources, so the LEDs just provide a different bias voltage at the cathodes.

Since the Crack is directly coupled, we don't want the plate voltage to change, so we adjust the current to bring it back down where it should be.

Unfortunately those LEDs I picked don't have a current vs. voltage graph to reliably predict the exact forward voltage, but it's likely close enough to be in the ballpark.

[Jamier]

So, for what it’s worth I tried another experiment. Instead of Crack to Q, I put another preamp in front of the Crack and then, Crack to the MoFos. The first preamp is a Lector ZOE (Italian, Bonzo should love that). Now I’m getting more power from the MoFos without the distortion that I was getting when the Q was in the chain, the distortion that I was mistaking for fading batteries, not the clipping gain stage problem that it was (I think).

Jamie

[grufti]

Unfortunately those LEDs I picked don't have a current vs. voltage graph to reliably predict the exact forward voltage, but it's likely close enough to be in the ballpark.

How about 2 HLMP-6000 LED's in series instead of the 3V red LED that you suggested? Would that work?

[Paul Birkeland]

Sure, that would work too.  If you have to order the new R1 resistors anyway, a different LED doesn't seem to be as problematic.

[Jamier]

I really don’t want to screw with my Crack as it is dead quiet and works flawlessly. I was thinking about building one dedicated for this purpose ( of driving the MoFos ). I noticed the Mainline is on sale. What is the V swing of it? At 999.00 it seems like one hell of a deal.

Jamie

[Jamier]

When is Cracktober? Did that already happen?

Jamie

[Doc B.]

Instead of Cracktober we did Doctober - almost everything was on sale at the beginning of the month. That was our main special offer for October. The current Mainline blowout sale is a deeper discount than was offered during Doctober.

[Jamier]

Are there any 120V versions left and what is the V swing on a Mailine.

Jamier

[Paul Birkeland]

On the "High" setting, there is enough voltage for your follower amp in the low power version, but not quite enough to drive the full power configuration all the way.  The Mainline is a very popular kit for this specific application. 

[Jamier]

Shopping for fans. Lot of choices. I'm overwhelmed at the number of choices. What Db rating will be more or less inaudible provided it doesn't add the noise of cavitation against the chassis?

Jamie

[Deke609]

Jamie - I picked up a couple of "Silent Wings 3" 140mm fans (non PWM) that run on anything from 5VDC to 12 VDC. At full power (12 VDC) they are spec'ed as max 15.5 dB(A). I tested one last night at full power and could barely hear it operate when it was held in in my hand in the open. At 9V I couldn't hear it at all.

And I just have to ask: "cavitation"?  I looked it up and still can't make sense of it - something to do with pockets of pressure drops caused by turbulence?

cheers, Derek

[Jamier]

Derek,

  What is the CFM rating? Not that I know what CFM rating I need but it will give me some reference point to start with. Cavitation,as I know it, is the disorderly flow of air or liquid when it does not move smoothly through a pathway (i.e. the openings in the bottom of my chassis) thus, resulting in noise. I don't generally like fan cooled amps as everyone I have ever had could be heard. But, I think PB is right on this one. If I don't use a fan I will need to use an even larger heatsink and the one I have now is already pretty big. It is quite astounding how much heat one MOSFET can produce.

Jamie

[Deke609]

[Edit: Correction! Mixing up my specs: 100 CFM @ 1000 RPM - 1000 CFM would be insane]

1000 CFM at max. The heatsink datasheets I looked at all dealt in feet per minute or second, or some such - don't remember. But there are online calculators for converting CFM to the unit of measurement you need for using datasheets to estimate the additional cooling effect you'll get from forced air.

You might also consider using a small fan - e.g., 40 cm and installing it so that blows directly against the long side of the heatsink at issue. There's a guy of DiyAudio - nikosokey - who did this and reports good results.

cheers, Derek