Followers and Quickie

[Jamier]

I’ve been using a Q in a gain chain to drive a MOSFET follower amp. I’ve noticed(at least I think I have) that the batteries drain faster in this application than they do when I’m using the Q to drive a conventional amp with a gain stage. Do Followers draw more from the preamp than conventional amps with  an input stage? It sort of make sense that they do? I have the Q in the last position right before the power amps

Jamie

 

[Paul Joppa]

The Q operates entirely in Class A, with constant current draw. The load it drives should make zero difference in battery life.

Sadly, batteries are one of the most frequently faked or mislabeled products around. So it's quite possible that batteries with the same label are not actually what they say they are, and you may have just found some cheap badly made batteries.

 

[Jamier]

PJ, thanks for your reply.

Jamie

 

[Bonzo]

Sorry for the OT question: do you mean you have a power follower amp (i.e. something with zero gain)?
If so, may I ask which model and how do you rate the quickie as the only "gain factor" of the chain?

Thank you!

 

[Jamier]

Bonzo,

    The amp(s) are MoFos (the boards are available @ diyaudio). They need a preamp that can swing a lot of voltage and I don’t have a preamp that can do that so I used my Crack followed by a Quickie to get the gain I needed. Based on my experiments a Q with PJCCS has nearly as much gain as Crack with Speedball so the two combined gave me enough gain to drive the amps. I don’t think the combination drives them to full power but it gets close enough. The amps are real simple, single stage, single ended and sound pretty good.

Jamie

 

[Paul Birkeland]

If you have the amps setup to make ~22W with the higher voltage power supply, then you need ~13V RMS or so at the inputs.  A Speedballed Crack will easily provide this with 1V RMS or so from a source. 

The Quickie cannot swing this much voltage because the plate voltage and batter voltage only differ by about 10V, so you're limited to about 7V RMS (probably a tad lower in reality) before your waveform clips asymmetrically.

I would recommend playing a 60Hz tone through your source and measure the AC voltage coming out of it with a meter to get a good idea of how much gain you need, and we can help you set things up so you'll get the voltage you need without clipping.

 

[Jamier]

I built the T version, around 12W. I’m going to get the 193V inductors and the 24V supply and try that with just the Crack.

Jamie

 

[Paul Birkeland]

12W is 9V RMS, so still too much to ask from a Quickie.

What are your source components?

 

[Jamier]

Mac PowerBook into a Cary Dac-100T

Jamie

 

[Paul Birkeland]

That's a ~2V out DAC, so plenty of output there.

Part of the issue is that the Crack won't take more than about 1V RMS of signal before you clip the input stage. 

You could replace the HLMP-6000 LEDs with 696-SSL-LX5093BHD LEDs that are 3V instead of 1.5V. 

To pull the plate voltage back where it should be, you'd want to set the R1 resistors on the front C4S board to 430 ohm resistors.

3V LEDs will let about 2V RMS into the Crack, and you'll get more like 25V RMS out.

 

[Bonzo]

The amp(s) are MoFos (the boards are available @ diyaudio).

Thank you very much, I was intrigued by that design too (I have two suitable chokes too).
Good to know that a crack could be a suitable Pre for such an amp!

 

[Jamier]

PB, is that a change to the LEDs on the speedball boards and the 9pin socket or just the SB boards?
Jamie

 

[Jamier]

Bonzo,

    If you build the MoFos use the same Avid heatsink that Mike used in the examples or something of equivalent size. Also, create openings under the fins and raise the chassis enough to allow adequate airflow.I built these with that big Avid sink and thought it was probably way more sink than I needed, I mean it’s just one little MOSFET, right? Well I couldn’t believe how warm they get.
I’m Going to build the 193V version and I’m hoping that heatsink is going to be enough.

Jamie

 

[Jamier]

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

Jamie

 

[Paul Birkeland]

PB, is that a change to the LEDs on the speedball boards and the 9pin socket or just the SB boards?
Jamie

Just on the 9 pin sockets themselves, the LEDs on all the boards need to all stay the same.

 

[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?