Bottlehead Forum
General Category => Technical topics => Topic started by: Jamier on October 13, 2019, 03:23:44 PM
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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
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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.
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PJ, thanks for your reply.
Jamie
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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!
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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
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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.
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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
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12W is 9V RMS, so still too much to ask from a Quickie.
What are your source components?
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Mac PowerBook into a Cary Dac-100T
Jamie
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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.
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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!
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PB, is that a change to the LEDs on the speedball boards and the 9pin socket or just the SB boards?
Jamie
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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
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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
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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.
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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.
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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
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@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.
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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
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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?
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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.
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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
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When is Cracktober? Did that already happen?
Jamie
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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.
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Are there any 120V versions left and what is the V swing on a Mailine.
Jamier
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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.
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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
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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
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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
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[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
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Are there any 120V versions left and what is the V swing on a Mailine.
Jamier
Yes, there is at least one 120V version of the Mainline left.
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Doc,
Thanks for that update, but PB is saying that the Mainline won’t quite drive the big MoFo version. Is there a mod, like the one he was suggesting for the Crack, that will increase the V swing without severely altering the character of the amp. I would not want to change an amp like a Mainline, irreversibly, anyway.
Jamie
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PB,
How short of full power would the Mainline come up when driving the 22W version. If it’s close I’m interested. And in the end if it doesn’t work out, at least I have a Mainline, right? It would drive my F4 well, right?
Jamier
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The F4 is usually the companion product paired with the Mainline as a power amp buffer.
I think you'd make about 14W. Have you evaluated the realistic power handling of the speakers you're using?
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PB, I use speakers that are mostly built by myself. Most of these operate well, outside of that figure. I listen semi nearfield so large output is not needed but the 20W rating associated with the big MoFo would be great for some of them. Several of these speakers are designed around 7 inch Seas midwoofers which do have good thermal compression and power handling capabilities. One of my dilemmas about the MoFo is how to increase the Current in order to drive the 4 ohm MTMs that I made designed around Seas ER 18s. I’m wondering if using the 193V inductor will accomplish this goal.
Jamie
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Increasing the power supply voltage isn't going to help with 4 ohm speakers. Increasing the idle current as much as possible would be a good idea, which would mean the beefier chokes and the same power supply. The 2.5A limit implied in the article will be your limiting factor, not the swing from the Mainline.
You could also investigate paralleling pairs of boards to increase current availability.
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PB, the increased idle current is what equates to the increased heat, right(?), so I still need the fan (or a bigger heatsink), even with the smaller SMPS, right?
Jamie
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Basically the whole DC rail appears across the fet, so you multiply that by your idle current and you get dissipation.
If you go from 12V DC to 24V DC at the same bias current, then you double the dissipation through the fet.
If you go from 12V DC 1.25A to 12V DC 2.5A, then you also double the dissipation through the fet.
If you go from 12V DC 1.25A to 24V DC 2.5A, then you quadruple the dissipation through the fet.
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So based on Mike's numbers, I'm going to get about 50% more dissipation, which means probably yes to at least some fan assistance( my idle current is 1.7 A @ 19V ) Thanks PB.
Jamie
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Also note that my power estimates are presuming that whatever voltage coming into the amp can be swung successfully by the MoFo. Personally I would use the 193V if your budget allows.
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Derek, who did you buy those fans from? Amazon?
Jamie
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Derek, who did you buy those fans from? Amazon?
Yup. I got the non-"PWM" version. I'm not tech-savvy, but I think PWM involves powering stuff from the motherboard. My fans came with a 3-wire/3-pin socket. I cut off the socket. Somewhat frustratingly, the wires are not color coded, so I had to experiment by process of elimination to figure out which was DC+ and which was DC-. It will only work one way. Reversing + and - makes the fan whir and sputter. I think the third wire is for a sensor or something - connecting it to power does nothing. But you could just as well google the 3pin socket to figure out which is which. But then you'll have to take the socket apart to keep track of which identical looking black wire is attached to which pin. Worst case scenario: it will tkae you 10 min to figure out which is which.
And note the fans are pretty big - 140mm. They also come in a 120mm version. And you could always get smaller fans from another manufacturer. I hit on the "Silent Wings" after a quick google search for "quietest computer fan" - there are other really quiet fans out there. But I didn;t feel like fussing for hours just to get 1 or 2 less dB.
cheers, Derek
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Derek, How will you power the fans? Battery, or some simple PS? 140mm is probably about right. the heatsink measures around 5 1/2 or 6 inches square, more or less. Actually it's about that tall too, so think of a 6 inch cube shape with fins, sharp, hot fins. I think the fan will be a good upgrade as long as it's not too noisy!
Jamie
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I haven't decided yet. I am going to mock up the BeePre rebuild using plywood to play around with layout to eliminate/minimize noise. I'll test various options then. Battery is obviously the safest bet noise-wise, since the power supply wouldn't be connected to either the amp wiring or mains. But some folks on DiyAudio who've used fans report no injection of noise running the fans off the amp filament supply. B/c I'm installing separate filament supply transformers, the existing 6.3V secondaries can be repurposed - so I'll try running the fans off of those (after doubling/rectifying and smoothing) and compare with battery power. If I can't hear or measure any additional noise, I'll go with using the 6.3V windings. But if that proves noisy, I'll try a separate trafo (10 or 12V). I might even try (being the crazy guy that I am) using a C4S in from of them to isolate them - but this assumes that isolation of the C4S (which I assume is function of impedance to AC - but don;t know) works both ways: PS and load each isolated from each other. Not sure if it does work that way or is instead one-directional.
I think I may also install a pot to control dc voltage/fan speed. To be even fancier, one can install a temp sensor or thermistor that controls fan speed. I'm thinking about that, but will probably leave it as something fun to play with in the future.
cheers, Derek
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PB,
I pulled the trigger on the Mainline, figuring that it can serve a multitude of purposes for me.
Will it have the same input clipping issue (1V) like the crack?
Jamie
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The Mainline won't take much more than 1.1V, but it has loads more gain than a Crack does (especially with the impedance switch set to High).
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Do I need to find a way to reduce the output of my DAC, or use a different DAC?
Jamie
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Do I need to find a way to reduce the output of my DAC, or use a different DAC?
I would set the output of the DAC all the way up and just use the attenuator on the Mainline as your primary volume control.
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So, I know this sounds like a pretty stupid question but, do I want to set up the fan to blow air through the Heatsink or draw air through it? I'm thinking drawing through might be more quiet.
Jamie
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I would blow the air onto the heatsink.
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PB, do you know the V output of an rDAC? It seems to be a better match with the Crack. I installed an Noctua Redux 900 fan in one of the MoFos. The fan cooled side is running 12-15 degrees C cooler than the other so I’m going to put the same fan in the other amp and the install the 193 Vs.
Jamie