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Old 2nd April 2013, 05:24 AM   #1
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Default How much power for an initial experimentation

I have mostly built tube guitar amps, and one of the draws for solid state hi-fi has been that experimentation can be done a lot cheaper than with tubes, as tubes require a big power transformer no matter what the wattage, and the ever expensive output transformer. Not to mention the tubes themselves.

I would like to build my first working amp. This thing will be a breadboard affair, except for the output transistors which will be on some heatsinks that I scavenged out of an old guitar amp.

So how many watts should a first attempt be? In guitar, the answer is usually a lot less than you might think, but in hi fi, I suspect I want more than I think. I was thinking maybe 20 watts for a start. Using an inexpensive set of Yamaha speakers from a pawn shop (nx-s75's), would 20 watts be loud enough with a enough headroom for me to have something usable? Rock, country, and classical would be the main program material, with occasional metal.

Also, what driver and output transistors should I get? I want to get something that can be scaled up for more higher powered future attempts. The small signal transistors will likely be 3094/3096s since I have lots of 'em

Any thoughts?
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Old 2nd April 2013, 06:15 AM   #2
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A 20watt amp, that too on a breadboard will be of no use for future scalability. In fact, there is no upgradation possible in the solid state except may be to increase output drive by paralleling the power devices.

I know you think of 20Watts as you know the tubes but is too less in the SS terms except if you want to go for class A.

If you want to go for breadboard version of the ss, better go for chipamps although I wouldn't recommend them for serious listening.

Upgradation and improvements on one base amp are not feasible and you will have to build and build newer ones till you are satisfied or tired of.

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Old 2nd April 2013, 02:21 PM   #3
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By scaleability I just meant what power transistors should I get that would work ok for a low power amp, but also would work in reasonably higher powered amps. I just don't want to make more than a couple mouser orders here.

If I build this "experiment" on a breadboard, of course I can make upgrades and improvements.
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Old 2nd April 2013, 04:22 PM   #4
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2N5551/2N5401 small signal
MJE340/350 Vas
MJE15032/33 drivers
MJ21195/96 outputs
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Old 3rd April 2013, 12:35 AM   #5
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@ djk: Couple questions about those transistors. Ok, maybe a few.

What is it about the 5551/5401 that makes it better than 3804/3906? All my books seem to use the 5551/5401 as you suggest, but a quick glance at the datasheet is not showing me what the key difference is.

About the mje340/350 for the VAS. Why such a high voltage/current component here? Or is there something else at play. Also, if I do a multi transistor VAS like an emitter follower driving a cascode, would I want the same part everywhere?

The MJ21195/96 looks like a pretty serious part! Looks like one could build a decently powerful amp with just a pair of those for output. I notice in a lot of pics of builds the output transistors are some sort of to-220 or similar packages in parallel. What would those typically be?

How about current sources and bias spreaders? Is it sufficient that they handle the voltages/currents or are there other criteria.
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Old 3rd April 2013, 03:00 AM   #6
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Quote:
Originally Posted by Dan Moos View Post
In guitar, the answer is usually a lot less than you might think, but in hi fi, I suspect I want more than I think.
Not that much, really; there's about a 6dB difference in efficiency between pro and hi fi speakers and a 6dB difference between dipole peaks and closed box---call it 10dB between an open back 12" cab and a typical closed box hi fi speaker. Means 1mW nominal for 60dB listening position SPL with a typical home audio speaker build versus a couple hundred microwatts from a guitar amp being used at bedroom volumes.

Either way an LME49600 or LMH6321 provides plenty of crest factor headroom, though having a few watts can be nice at times depending on the neighbors. You can go discrete on the output devices if you want---look at Sanken STD03s for bias tracking---but it's extra cost and complexity for lower performance than a composite chip amp with a good op amp in the control loop---search for nested feedback in the chip amp forum. This approach works up to a few tens of watts, with discrete outputs making more sense above that---look at the LME49811 as a controller.

Of course, if your goal's to muck about with amp innards then ICs aren't so useful.
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Old 3rd April 2013, 03:31 AM   #7
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Yeah, I suspect a chip amp isn't what I'm after. I want to "muck with the innards" as you say!

My comment on guitar amp wattage requirements reflects the fact that for a lot of us, headroom is a bad thing with tube guitar amps, so low wattage is used so that the power section of the amp can be distorted at relatively low and non-painful volume levels.

For instance, the amp I currently gig with puts out maybe 9 watts cleanly, but will do 30 with everything up. This would be considered a 30 watt guitar amp. I doubt anyone would describe a hi fi amp as 30 watts if it was clipping as heavily as this guitar amp does at that wattage!

I was wondering if 20 watts into an average efficiency home stereo speaker would be enough headroom to enjoy listening to. I just want to have it loud enough so that I can discern any improvements I make as I go. The 100 watt amp that I will eventually build will be the result of these early lower wattage learning attempts.
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Old 3rd April 2013, 04:22 AM   #8
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If you are after scalability, as in increasing the power dramatically, it won't happen by just substituting parts with higher ratings and adjusting other components to suit higher voltages etc. This usually involves a complete redesign, new power supply, caps and all the expensive stuff and experimenting all over again to reestablish similar performance at the higher voltages and current throughout the design.

Within limits, simply tacking on more output devices and heavier drivers allows more current output capability and that does enable more power by driving lower impedance loads such as multi-speaker arrays.

Practically speaking though, a 100W amp. will be very little different in cost to a 25W one. There is almost no point trying to start low and scale that up. However, if you start at 100W and scale up to 400W, there will be quite a cost difference as the major parts and physical size start to be significant. This also has a decided advantage in using the same, hopefully rugged device types of the lower power units and that maintains the same cramped headroom, if that's desirable for guitar use.
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Last edited by Ian Finch; 3rd April 2013 at 04:26 AM.
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Old 3rd April 2013, 04:24 AM   #9
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Quote:
What is it about the 5551/5401 that makes it better than 3804/3906? All my books seem to use the 5551/5401 as you suggest, but a quick glance at the datasheet is not showing me what the key difference is.
Look at the Vceo breakdown voltages. 2N3904/3906 is only 40V. 2N5551/5401 is much higher.
Quote:
About the mje340/350 for the VAS. Why such a high voltage/current component here? Or is there something else at play. Also, if I do a multi transistor VAS like an emitter follower driving a cascode, would I want the same part everywhere?
The VAS transistor typically has to endure rail to rail voltages, so this means they must be able to survive without overvoltage breakdown. Also the driver stage(s) must handle rail to rail voltage as well. With hollow state you might get away with pushing the voltage limits of the device a bit but with solid state you will usually end up with the magic smoke being released, and it can't be put back in. Make sure you take into account Pd derating curves in the datasheet for the devices. Pd max is not really Pd max.
Quote:
The MJ21195/96 looks like a pretty serious part! Looks like one could build a decently powerful amp with just a pair of those for output. I notice in a lot of pics of builds the output transistors are some sort of to-220 or similar packages in parallel. What would those typically be?

How about current sources and bias spreaders? Is it sufficient that they handle the voltages/currents or are there other criteria.
MJ21195/96 can produce ~100Wrms with class AB bias. Being new to solid state I would suggest you shoot for ~25W if you intend to use class AB bias. Larger amps could be problematic for a first discrete design. If you intend to start screwing with the 'inards' and cause a failure of the output stage (quite easy to do), a larger amp with more expensive output devices will set you back more $. 25W can be done with just one pair of TO-220 transistors. D44H11/D45H11 is a nice pair, cheap and available; I have used them to build an amp of that size before and can be easily breadboarded. If you use a CCS to bias the VAS it will also have to swing rail to rail so it should have the same adequate Vceo. 25Wrms @ 8R has a peak voltage of only 20V, so a +/-22V supply should be adequate taking into account rail voltage sag under load. This opens up many more suitable transistors that are available to use. If you don't already have a power transformer, for a cheap start you might look into the 12.6V -0- 12.6V @ 2.5A transformers available a Radio Shack for $10, at least they use to be $10......... One of these will give you about +/-18V after rectification which should do about 20W @ 8R.
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Last edited by CBS240; 3rd April 2013 at 04:53 AM.
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Old 3rd April 2013, 04:35 AM   #10
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IMO, there's a sweet spot with power supply rails of about +/-40 volts. That should get you 60 or so watts with 100 volt or so rated output devices, more if you parallel for current and use 4 ohm speakers. I'm not sure there's any advantage cost or difficulty-wise building for less than 40-60 watts.
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