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Old 3rd November 2013, 03:04 AM   #1
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Default 75 Watt/Channel Monoblock Custom Amp

After reading more than the average beginner reads (I've bought four books & checked out 7 from Library...plus the internet), I realize that it may take a long time before I can custom design my own amplifier and build it (possibly years). That said however, I wanted to get your initial thoughts on what I currently imagine my final ultimate amp design will end up being (just an abbreviated summary of some specs).

1) 75 watt monoblock (75 per amp for 75 per channel)
2) push pull triode mode ( $1,000 allocated per block for output tubes)
3) 10Hz to 40Khz +/- 1 db
4) high signal to noise
5) Class A
6) toroidal power supply

So I'd love to get your initial thoughts on my long term goal. I know that I have many short term goals to achieve before I get there but your thoughts will probably help me adjust the beginning part of the quest/journey. In other words, I might be very far from Taiwan or Peru but to increase my chances of ever arriving at one of those countries, my first steps will depend on which country I'm heading towards.

I'm an extreme reader and it makes my journey different than some of my friends who learn better by diving in first. We're all different and I feel like I do better in complex projects if I read a lot first.

Your thoughts are valued and closely considered.
kind thanks
Eddie
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Old 3rd November 2013, 03:35 AM   #2
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The 75 W. part is relatively easy. Use PP Class "AB2" KT88s in either UL or full pentode mode. Tubelab's PowerDrive is what you use to take the KT88s into a mild positive grid current regime. Blocking distortion is prevented by this kind of circuitry.

Class "A" PPP triodes will give you all sorts of grief. Tube matching, current hogging, driver circuitry difficulty, and HEAT are some of the problems to be resolved.

Check out the offerings of AnTek, as a source of affordable toroidal power transformers.

BTW, bandwidth down to 10 Hz. is (IMO) not a good idea. LP warp noise lies there.
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Old 3rd November 2013, 10:13 AM   #3
AJT is offline AJT  Philippines
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hmm...have you been building tube amps before?

if not better get a kit as a first project....there are many sellers of sucj kits...

the goal there is to get you some practical experience, in case that amp is your first build...

better to start small and work your way up to big....
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Old 3rd November 2013, 11:32 AM   #4
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Perhaps you could elaborate on your requirements?
Do your speakers need this much power and do they go down to 10Hz?
PP 845 triodes would get you there, but that is definitely not a project for a first (second, third...) build.
Perhaps you can try some valve amps in your system so you hear what you like and need.
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Old 3rd November 2013, 02:38 PM   #5
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A few general pointers (modestly, in the presence of tube design dignatories here),

Frequency response: One does not primarily design for a wide frequency response - it comes along while achieving other goals. One tries to get an open-loop (i.e. without negative feedback) response of some 20 - 30 Hz to say 15 kHz, then apllying NFB usually results in a bandwidth of < 10 Hz to > 50 kHz.

Class A: The 'pure class A' phrase is an overrated myth; class A is not necessarily better than class AB while unnecessarily wasteful of power/valve life. It originated from early problems with achieving low cross-over distortion (clean output at low level). That is not a problem with any knowledgable designer any more.

Toroidal power transformer: Again rather overrated, one can choose this on the basis of cost and availability. I use C-core transformers because they are more easily obtainable/wound (in RSA); somewhat more efficient and smaller than E-I lamination types.

Other than that, as Eli and others said. Much of the success is reached through proper, neat construction and a good design, of which many exist. (And an awareness of safety in the presence of several hundreds of volts!)

Good luck! We all started somewhere; hopefully this is the point for you.

Last edited by Johan Potgieter; 3rd November 2013 at 02:40 PM. Reason: Typo
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Old 3rd November 2013, 05:21 PM   #6
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75W Class A parallel triode mono-blocks?! You are one ambitious fellow... repeating what AJT said, do you have any electronic or DIY experience?
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Old 3rd November 2013, 06:00 PM   #7
roline is offline roline  United States
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pete millets big red pcb will do 125/channel+ depending on power supply and output iron choices.
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Old 3rd November 2013, 07:12 PM   #8
wicked1 is offline wicked1  United States
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I'm a big fan of Mr Millets mono PP driver PCB. It's similar to the big red board, but w/out the output stage so you can more easily use whatever tubes you want. PPP triode kt88 or whatever, is no problem. You still get to do some amplifier design to set it up for your tubes and transformers, and there's a high chance of success because Pete's set some design parameters and done the layout already.

I've built a 2 stage, all triode, no feedback amp, as well as a 3 stage, parallel push pull, pentode, global and local feedback amp with his driver boards.
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Old 3rd November 2013, 09:36 PM   #9
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Don't forget that you will need the basic tool set: variac, soldering station, 2 DMMs, signal generator, oscilloscope, diagonal cutters, pliers, screwdrivers, hole punches, dummy loads and an assortment of resistors, capacitors and hardware.

I suppose that beginners forget that building the unit is only half the job. Once it is built you need to be able to troubleshoot, repair, rework, and test to ensure it is working properly. That is another skill set.

Read both of Morgan Jones' books and then consider your task. There are no short cuts. You will need skill and knowledge to accomplish your goal.
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Old 3rd November 2013, 09:43 PM   #10
DF96 is offline DF96  England
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When I was first learning electronics and knew just enough to confuse myself I used to dream up 'ultimate' amplifiers/receivers etc. Later on I realised how crazy they were, as things I hadn't even heard of then (e.g. loop stability theory), let alone understand, would trip them up big time.

It's nice to dream, but don't waste too much time on it. And don't pursue too many audiophile myths. Instead, learn to distinguish fact from fiction. Build something and debug it. Books are good (I read a lot, and by nature I am a theorist) but electronics is engineering, not pure science.
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