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loftin white 801 amp

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... The final cap to ground after the PS choke, as suggested by Darius, does make a significant difference in residual hum. Without the cap, I hear hum (maybe 3mV or so), with the cap it's less than 0.5mV...

Sheldon,

I have started building this amp. Since the hum level is so low, do you think there would be any benefit from using a regulated power supply or additional filtering?

TIA,
 
Sheldon,

I have started building this amp. Since the hum level is so low, do you think there would be any benefit from using a regulated power supply or additional filtering?

TIA,

Cool, I just spotted your post. I don't see the need for regulation or additional filtering. Give it a try this way first, and leave room for future playtime, if you like. I think you will find the amp very quiet as is. You will need DC on the filaments. The Tentlabs heaters I'm using work fine. But you might check out Rod Coleman's nice design before you spec. out your power tranny. He's offering kits, I believe. BTW, I have an extra power transformer just sitting here.

Sheldon
 
What is the sound quality like and how does it compare to other systems you have used?

It sounds nice. Very clean. I'm using it now as my headphone amp, running 64Ohm phones on a 32Ohm tap. It has surprisingly good clean bass response too. I've used it on the compression driver for my Unity horn too, and I liked it a lot. But, the 801 amp only puts out around 1 watt, so it won't shake the walls. It could be very nice in a small desktop system.

I don't like a lot of distortion, even euphonic distortion, so I build only amps that meet that criterion.

Sheldon
 
Sheldon - thanks for the feedback. What are your thoughts on choke loaded direct-coupled designs (free-lunch, monkey direct reactance types) vs. those without? (e.g. loftin white or choke-less free-lunch.)

I've been experimenting with basic circuit models based on the free lunch / monkey amps, they model fine without the choke but once I add the choke it seems to compromise the signal response. What happens is that the power output increases but the signal output becomes distorted because the choke resists any sudden changes in current. To my mind this would compromise the fidelity of the amp on complex music? I wonder if the extra power worth this compromise? I don't see see any benefit to the choke if one has high efficiency speakers that do fine on low power amps.

This leaves me wanting to try something like the free lunch amplifier without a choke or the loftin white (which doesn't have a choke.)

Am I misunderstanding the benefits of the choke in the free-lunch / monkey / direct reactance amps?
 
Sheldon - thanks for the feedback. What are your thoughts on choke loaded direct-coupled designs (free-lunch, monkey direct reactance types) vs. those without? (e.g. loftin white or choke-less free-lunch.)

The original L/W amp was designed to minimize the cost of the unit. At the time, large value high voltage capacitors were not available in at a reasonable cost or small enough size. The elimination of the coupling capacitor was not a sonic issue, but a cost issue. What they came up with was a very elegant solution to using a minimalist power supply. The original was based on a pentode for the input, as they needed much more gain than we need today for a power amp.

Darius adapted the elegant noise canceling approach to be used with an input triode. The result is a two stage amp with no coupling capacitor and no cathode capacitor, which means reduced distortion from the input tube. And not an electrolytic cap in sight, as only reasonably sized film caps are needed for the power supply.

I don't think most people really understand the amp. It looks so simple but takes a little thought to grasp the whole concept. However, it does have limitations. Because the operating conditions of the input and output tubes are tightly linked, the number of possible combinations are much more limited than with other two stage amps - especially if you want to build with a single triode per stage. If you add Darius's bootstrapped follower, the field opens up more, but is not unlimited.

On the other hand, once you get a set of tubes and operating conditions optimized, the amp requires no adjustment over the life of the tubes. Set it, forget it.

The Free Lunch and Monkey are direct coupled, but that's where the similarity to the L/F ends. Both use the bypassed output cathode resistor as a power supply for the input stage. A choke loaded input stage is required to get the required voltage swing at the input plate. Neither has any power supply ripple canceling. The DRD design employs the same method to feed the input tube but bypasses the output cathode to the B+. That will provide some additional PSRR for the amp. It should be reasonably effective, but it's not tuneable, as is the L/F design. In all three designs, the input cathode is AC bypassed to ground.

I've been experimenting with basic circuit models based on the free lunch / monkey amps, they model fine without the choke but once I add the choke it seems to compromise the signal response. What happens is that the power output increases but the signal output becomes distorted because the choke resists any sudden changes in current. To my mind this would compromise the fidelity of the amp on complex music? I wonder if the extra power worth this compromise? I don't see see any benefit to the choke if one has high efficiency speakers that do fine on low power amps.

The power increases with the choke, because you increase the swing of the input plate above it's supply voltage and can drive the output tube to full output. The distortion at equal outputs shouldn't increase with the choke. In fact, compared to a resistor loaded input tube, it should be less. And yes, the choke resists sudden changes in current. In that sense, it functions as a constant current source (which is why the distortion should be less). But, unlike a CCS, it can store energy and drive the plate typically to about twice the voltage at the choke input. Even if you don't push the amp, the choke has the benefit of reducing power supply ripple at the input plate.

Sheldon
 
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