this is the 1983 version of krell..i will use bc546/556 for inputs and mje340/350 for the driver of mje15030/31 and finally for the outputs i will use mjl1302/3281 or njw1302/3281..because i dont have mjl21194/21193 now.someone says mjl21193/94 is one of the most reliable transistors for class a mode..but i will try quarter class a mode.
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umut1001 said:
Yes it can, but you do not need to use four output devices if you are going to use class AB. Bias output at about 120 mA, it will run warm but stable.
You will not hear unmatched transistors, use the 1503/1 as you bought them, adjust off-set to minimum and it will be fine.
You will find off-set drift with temperature so do final adjustment when amp is at stable temperature after say 1/2 hour or so.
umut1001 said:
Since you will operate at a moderate supply, I find that the BD139/140 has a "nicer" sound as VAS. The purpose of many output devices is to spread SOA over multiple transistors, this is especially valid for long term or DC use such as class A.
When running class AB then SOA is dependent on instantaneous current, the demand on the output devices does not even approach the continuous demand of class A.
Finally to check bias setting and correct operation of Vbe multiplier, measure the current through output devices at idle, then heat the heat sink with heat gun. When heating then current through devices should drop the hotter it gets, if it increases then you have a possibility of thermal runaway situation.
The Pre 1983 Krell is quite a nice sounding amp and would appeal to the general listener, it has a very solid bass and good HF presence, besides it is an easy amp to build with very few quirks. It gives the impression that is sounds very powerful.
The Pre 1983 Krell is quite a nice sounding amp and would appeal to the general listener, it has a very solid bass and good HF presence, besides it is an easy amp to build with very few quirks. It gives the impression that is sounds very powerful.
umut1001 said:
The circuit you show here is pre-1983 and the KSA50 that started the KRELL as an audiophile amp of its time. It is an enjoyable amp to listen to but it can over emphasize electronic generated music in the high end.
Output impedance is only a few mili-ohm and this would support its "solid" bass due to the highly dynamic control of the speaker. There is nothing critical about the design and to use the silly values of resistors shown on the circuit is totally unnecessary, use readily available resistor values, you will not hear or measure any difference.
Make sure your supply capacitance is large preferably 22mF per rail and 35 amp bridge, which will give you very pleasing results.
umut1001 said:
You can use TIP 41/42C but they sound a little harder. If you plan to use it at the low voltage end (40V/Rail) then the BD139/140 is a good sonic choice and you can make the VAS from MPSA56 and MPSA06.
I have built that exact circuit with the transistors mentioned in the past for a non-electronic friend many years ago and he still will not part with the clone even after 15 years. It is not a bad amplifier at all. It sounds powerful very dynamic and fast, although non of this can be confirmed by measurement because by design it is not really a fast amp, but that is the impression you get when listening and probably why it is so popular.
I think when Dan designed this amp he had very good marketing intelligence of what people would expect from the sound of a then "high-end" amp.
umut1001 said:
Vceo is 45V, a little on the low side if you are going to run that the supply voltage you suggest. But there is no reason that you cannot run the amp at lower supply. Only difference is you will get less output, besides heat sinks will cost less as less heat is generated. Another advantage is you could get away with a single output pair.
FYI: Just how tough is a KSA50?
My friend Neil and I are just completing two pairs of high bias KSA50 (+/-40v, 2.5A). The first pair were played at BAF2008, where I thought they did rather well.
Anyway we made each channel using a 650VA toroid, 40000uf, and 6 pairs of plastic output devices with 2ohm emitter resistors. The 2ohms was chosen to keep the drivers bias point about the same as the KSA50.
While doing initial testing we (accidentally) found that the amps were perfectly happy driving a ZD5 speaker in parallel with a CL60 thermistor, to the limits of the 3.15A slow-blo primary fuse (5-6A?)
We interpreted this as a result of the inefficiency of the speaker, so we replaced the fuse a few times to make sure...By the time the fuse blew (over and over) the amp was delivering ~35A peaks into the 1 ohm load into speaker and thermistor represented...it was the temperature of the CL60 that alerted me to the problem...
Not to be outdone, while bringing up one of the other channels I played it into a short for long enough that some copper strands that were otherwise hidden, started glowing in time to the music, alerting me to their presence...who knows how much current this was?
...and at no time in any of this did the amps give any audible sign of distress, they just played beautifully until the primary fuses blew...
Clearly You can't rely on hearing distortion from these amps to let you know there's a problem.
Stuart
My friend Neil and I are just completing two pairs of high bias KSA50 (+/-40v, 2.5A). The first pair were played at BAF2008, where I thought they did rather well.
Anyway we made each channel using a 650VA toroid, 40000uf, and 6 pairs of plastic output devices with 2ohm emitter resistors. The 2ohms was chosen to keep the drivers bias point about the same as the KSA50.
While doing initial testing we (accidentally) found that the amps were perfectly happy driving a ZD5 speaker in parallel with a CL60 thermistor, to the limits of the 3.15A slow-blo primary fuse (5-6A?)
We interpreted this as a result of the inefficiency of the speaker, so we replaced the fuse a few times to make sure...By the time the fuse blew (over and over) the amp was delivering ~35A peaks into the 1 ohm load into speaker and thermistor represented...it was the temperature of the CL60 that alerted me to the problem...
Not to be outdone, while bringing up one of the other channels I played it into a short for long enough that some copper strands that were otherwise hidden, started glowing in time to the music, alerting me to their presence...who knows how much current this was?
...and at no time in any of this did the amps give any audible sign of distress, they just played beautifully until the primary fuses blew...
Clearly You can't rely on hearing distortion from these amps to let you know there's a problem.
Stuart