I don't think you're missing anything. I may just not know the proper term. I'm still a relative noob. So, I defer to others' experience a lot. I commented on your question b/c I was in a similar situation when I chose to build a few of the Burning Amp variants. So, I can maybe help a bit. Technical stuff will have to come from those more experienced / knowledgable. I don't know what you mean by -ve. That's not you... that's me.
So we can get on the same page. If you go balanced (and use a "standard" build with store boards) you would need 2 output boards per channel. One for what is commonly called the positive phase and one for what's commonly called negative phase. I'm not sure those terms are proper, but they're the vernacular I've seen used.
You can also run more than 6 devices (one board) if you want. I've seen up to 12 (two boards) per phase, and I plan to try that. I have boards and big enough donuts for a total of 48 output devices across two channels. 12 per phase / stereo / balanced. Just add as many boards as you have heatsink / transformer capacity for. Right now, I'm running 6 devices per channel in standard single-ended stereo at a 0A5 bias (or so). Sounds fabulous.
A common balanced BA-3 build could have two chassis with one BA-3 input board, and two output / bias boards each.
Correct. However, you could (if you had the heatsinking and that's what you wanted) run two or three or more. I also think you could remove output devices from each board and tweak to taste, but that's your call.
Sure, but as I've said... that may be a pretty big chassis. Sketch out what you'd need to accomplish that. Do the math for the power you want, see how big the heatsinks may need to be... decide if you want any other doo dads like speaker protection or electronic soft start, figure out where they'd go in the chassis, and go from there.
Yes, but if that's the path you choose, be sure to read the thread. It should be fairly straight forward. One of the coolest things to me about the Burning Amps is that they're "modular".
You may need to explore the various threads a bit if a Burning Amp or some variant is starting to hold your interest. The design is extremely versatile. You can create about as much power as you'd ever want by running higher rails (with some tweaks) and/or just continuing to pile on the output devices in parallel for more current. My point is that as long as you have the heatsinks.... you can safely and without even going outside the spirit of the design run 32V rails and 12 devices per phase at 35W dissipation per device. Now.... you do the math for how big those heatsinks would be.
I think what may be giving you pause is your comments re: power. In almost all cases these amps would be limited in total output by the heatsinks... not the electronics. You want moooooore power, get bigger heatsinks, run 32V rails and jack up your bias until you've created a true "Burning Amp"... they'll be hot... thus the name.
Back to your very early question and a simple answer...
Pick any of the amps already discussed. You really can't go wrong. Once you've built one... you'll build more any way
Are you looking for something that can take a balanced XLR input
without necessarily a balanced output stage? If so, have a look at this:
http://www.firstwatt.com/pdf/art_ba2.pdf
Here you have the same output stage but a different input stage with a differential
pair, allowing a balanced input.
The output stage on its own is basically the F4 without the input buffer.
If your preamp has adequate gain and voltage swing you might consider just
building the F4. A stereo F4 can be configured for stereo, mono balanced,
and mono parallel operations. See p7-8 here:
http://www.firstwatt.com/pdf/prod_f4_man.pdf
The last few pages of the F5 turbo article sketches out ideas for a balanced
version:
http://www.firstwatt.com/pdf/art_f5_turbo.pdf
There are some threads on making a balanced F5. For example:
Balanced F5 question
Wow you guys are awesome!
Thanks guys great info really appreciate your help. All great anwsers.
Option 1.
Based on your feedback and the fact my preamp can drive 20Vp-p I think the F4 (with no buffer pre stage) looks like the best starting place.
I can run single ended test it out in stereo and then go mono xlr later and add another unit to drive up power per channel and have end to end balanced output etc. Super simple build as a first run at it. Start with 4U or 5U chassis.
Option 2.
Another option might be run just the burning amp output stage with two boards balanced per heat sink but reduce the number of output fets to half per board and run a channel on each heatsink. 4 fets per side of the balanced output stages. Probably need customized mounting for that though so not so simple build.
The first one looks like a safe option for me to start as i can always swamp stuff in and out later.
Thanks
Thanks guys great info really appreciate your help. All great anwsers.
Option 1.
Based on your feedback and the fact my preamp can drive 20Vp-p I think the F4 (with no buffer pre stage) looks like the best starting place.
I can run single ended test it out in stereo and then go mono xlr later and add another unit to drive up power per channel and have end to end balanced output etc. Super simple build as a first run at it. Start with 4U or 5U chassis.
Option 2.
Another option might be run just the burning amp output stage with two boards balanced per heat sink but reduce the number of output fets to half per board and run a channel on each heatsink. 4 fets per side of the balanced output stages. Probably need customized mounting for that though so not so simple build.
The first one looks like a safe option for me to start as i can always swamp stuff in and out later.
Thanks
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The f4 is the output stage of the BA-3. Easy to change to BA-3 (crippled F4) one output board at a later date. From what little I have read with two BA-3 front ends you can make it balanced without a lot of trouble. It is a great choice for the first FW clone. I am still not sure 25 watts will wake up those electrostatics but it all depends on your listening sound level. To make it an easy build buy 2 tubes of 25 output mosfets to get a better match and still go ahead and match them. You can always use the extra mosfets for another project. The cost is not that great considering the cost of 2 output tubes in a tube amplifier build. I converted my F4 to the crippled F4 using the BA-3 front end to have enough gain to use any linestage.
If you want to drive the output stage directly (or 'crippled F4', ie, F4 without input
buffer) you should find out the output impedance of your preamp and make sure
it's reasonably low.
I agree with wdecho's suggestion of getting two full rails of mosfets and match
them yourself, especially since you may build a second amp in the future. And
the mosfets price aren't bad...
buffer) you should find out the output impedance of your preamp and make sure
it's reasonably low.
I agree with wdecho's suggestion of getting two full rails of mosfets and match
them yourself, especially since you may build a second amp in the future. And
the mosfets price aren't bad...
Here are my spec's from manufacturers website on the output of my preamp.
I think this looks fairly good.
Balanced Output Specifications
Output Impedance: 20 Ohms.
Maximum Output Level: 20 Vrms balanced.
Frequency Response: 20 Hz-20 kHz, ± 0.025 dB, Ref. 1KHz.
THD+Noise: Less than 0.0015% @ 1KHz, maximum output level.
Dynamic Range: 105dB minimum, 20KHz bandwidth, Ref. 1KHZ, A-weighted.
Signal to Noise Ratio: 105dB typical, idle channel, A-weighted.
Crosstalk -105dB Right – Left, >-120dB Center-Left @ 20KHz.
When you say a full rail of mosfets do you have a suggested supplier? I have not ordered components in a long time.
Also what process do you suggest in terms of matching?
I think this looks fairly good.
Balanced Output Specifications
Output Impedance: 20 Ohms.
Maximum Output Level: 20 Vrms balanced.
Frequency Response: 20 Hz-20 kHz, ± 0.025 dB, Ref. 1KHz.
THD+Noise: Less than 0.0015% @ 1KHz, maximum output level.
Dynamic Range: 105dB minimum, 20KHz bandwidth, Ref. 1KHZ, A-weighted.
Signal to Noise Ratio: 105dB typical, idle channel, A-weighted.
Crosstalk -105dB Right – Left, >-120dB Center-Left @ 20KHz.
When you say a full rail of mosfets do you have a suggested supplier? I have not ordered components in a long time.
Also what process do you suggest in terms of matching?
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The output impedance is plenty low.
I don't know where you are located, but Mouser and Digikey are two well
respected suppliers. The output mosfets in question (IRFP240PBF/IRFP9240PBF)
come in rails of 25. So just order 25 of each. Getting a full rail means
the come from the same lot, which facilitates matching.
Here's an article on matching; the method outlined is pretty easy to do:
http://www.firstwatt.com/pdf/art_matching.pdf
I don't know where you are located, but Mouser and Digikey are two well
respected suppliers. The output mosfets in question (IRFP240PBF/IRFP9240PBF)
come in rails of 25. So just order 25 of each. Getting a full rail means
the come from the same lot, which facilitates matching.
Here's an article on matching; the method outlined is pretty easy to do:
http://www.firstwatt.com/pdf/art_matching.pdf
On a related note, I decided to build a single dual mono PS to power whatever First Watt clone I want to play at that time. Plan to have the amp modules as mono blocks connected to the PS box. I planned to use the DIYaudio store PS boards, which I already have. I am starting with the M2X and the Aleph J, but intend to also build the F6 and the BA-3 or F4 (balanced monoblocks). Would all of them work with 400VA x 18V transformers or would a different secondary voltage be better.
There is another reason to operate the output Mosfets at a higher voltage, beyond simply sending more voltage to the speakers. That is to let the devices operate at or at least closer to their "sweet spot." This is a combination of Vds and Iq that will work better in conjunction with the load on the output.
There has been much good discussion in some of the SIT related threads, but these big hocky puck style devices are not the only ones to benefit from more optimized voltage and current. What you can typically expect to notice is improved transconductance from higher Vds and/or higher Iq. This translates into what might be perceived as a more lively or forward sound in your system.
Empirical evidence suggests that the ACA, MoFo and F6 all benefit from higher Vds across the output Mosfets. We can expect the other large class A designs to show some improvement as well.
There has been much good discussion in some of the SIT related threads, but these big hocky puck style devices are not the only ones to benefit from more optimized voltage and current. What you can typically expect to notice is improved transconductance from higher Vds and/or higher Iq. This translates into what might be perceived as a more lively or forward sound in your system.
Empirical evidence suggests that the ACA, MoFo and F6 all benefit from higher Vds across the output Mosfets. We can expect the other large class A designs to show some improvement as well.
Check this thread to see which amps can be balanced https://www.diyaudio.com/forums/pass-labs/357078-amps-balanced.html#post6270103
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