[UntilThen]

Here are the Taylor graphite plates in situ.

This is a statement amp. Would love to hear it with my headphones and speakers. I have word that Sowter transformers are on it's way to Tomas for 15th July delivery.

 

[L0rdGwyn]

I finished the last PCB for my OTL design today and BOY, was it a doozy!

The PCB contains power supply components for both channels of the dual mono power supply including...

• Mains transformer snubbing components
• Mains transformer secondary protection fuses
• Current limiting resistors
• Full-wave bridge rectifiers
• Reservoir capacitors
• Time delay circuit components

So in goes high-voltage AC, out comes time-delayed raw DC before it hits the power supply regulators.

Some minor details about the time delayed aspect - there will be ~30 second delay from the time the amplifier is turned on before the high-voltage power supply is activated. During that 30 seconds, the tube heaters will be warming, such that the tubes are full ready to conduct at the time B+ is applied. A soft start is also included, so in total the amplifier will not be fully powered on until ~45-60 seconds after the power switch is flipped.

Why is this beneficial? For one, it reduces the possibility of any arcing of the 6AS7G power tubes on startup as there are never excessive grid-to-cathode voltages (a major contributor to the arcing experienced in these types of amplifiers). It also prevents any unpleasant sounds from voltage transients as the large 265uF output capacitors charge. And if you believe in cathode stripping, it is the gentlest way to apply power to NOS tubes.

Throughout the day as I worked on this PCB (and it did take all day), I intermittently went to my work bench to listen to this amp for motivation! Man, I think it sounds really good, so I got it done, I can order all of the final components tonight and finalize the chassis layout.

Coming into the home stretch on this design, hopefully can build it in a few weeks, then get some ears on it.

 

[whirlwind]

Kudos on the soft start, it is a great feature

A little peace of mind when using Tung Sol 5998....most bad things happen when the amp and tubes are being powered up.

 

[L0rdGwyn]

From a chassis design standpoint, here is what I have in store for this OTL amplifier.

Toroidal shield in the center, 265uF Clarity Caps will be mounted on the interior but protrude through the top plate (back left and right). 6J5 tubes in the front, 6AS7G types in the middle, sockets for both will be mounted on sub-plates, similar to my 801A amplifier. I've designed a "flower" pattern for the 6AS7G ventilation. As you can see, the amplifier will be very well-ventilated.

Small changes might be made, but it is almost done. It is a large amp, size as it stands is 13.25" x 15.25" x 3.25". Large caps, large amp

 

[chrisdrop]

From a chassis design standpoint, here is what I have in store for this OTL amplifier.

Toroidal shield in the center, 265uF Clarity Caps will be mounted on the interior but protrude through the top plate (back left and right). 6J5 tubes in the front, 6AS7G types in the middle, sockets for both will be mounted on sub-plates, similar to my 801A amplifier. I've designed a "flower" pattern for the 6AS7G ventilation. As you can see, the amplifier will be very well-ventilated.

Small changes might be made, but it is almost done. It is a large amp, size as it stands is 13.25" x 15.25" x 3.25". Large caps, large amp

Stop making me want more amps....

 

[L0rdGwyn]

Stop making me want more amps....

I'm excited to get one of these built and get some feedback on it. I am enjoying the sound, I hope others will too. To me, it has excellent transparency, dynamics, and detail retrieval. With the way the circuit has been designed, I also don't think you need very rare / expensive 6J5 / 6AS7G to get very good sound from the amplifier.

I will be using a 40mm RCA fester style bakelite knob. Have to import from Europe unfortunately. Everything has to be the way it has to be...

 

[mordy]

I'm excited to get one of these built and get some feedback on it. I am enjoying the sound, I hope others will too. To me, it has excellent transparency, dynamics, and detail retrieval. With the way the circuit has been designed, I also don't think you need very rare / expensive 6J5 / 6AS7G to get very good sound from the amplifier.

I will be using a 40mm RCA fester style bakelite knob. Have to import from Europe unfortunately. Everything has to be the way it has to be...

Will the new OTL have preamp capability?

 

[L0rdGwyn]

Will the new OTL have preamp capability?

No, I had not planned on it. Maybe I will consider it down the road.

 

[mordy]

No, I had not planned on it. Maybe I will consider it down the road.

Is it possible to just connect the headphone out to RCA jacks in the back to serve as a preamp?

 

[Tjj226 Angel]

Will the new OTL have preamp capability?

You can simply buy a headphone to RCA adapter and use it as a preamp.

Although I strongly advise against using any headphone amp as a preamp. A headphone amp will usually have much more noise, distortion, and phase shift over a purpose built preamp.

 

[L0rdGwyn]

Is it possible to just connect the headphone out to RCA jacks in the back to serve as a preamp?

You could do it that way, but really not the best way. A power amplifier (or whatever gear is being fed by the pre-outs) will typically have an input impedance >50K, which means the output capacitance of the pre-outs does not need to be as large as it is for driving a headphone. For instance, a f-3dB point of 10Hz with a 50K load would only require an output capacitance of 0.32uF. This amplifier has an output capacitance of 265uF. Why does that matter?

On startup, these caps need to charge. On the output, they will do that through the parallel resistance of the load (headphone or amplifier fed by the pre-outs) and a parallel resistor. This combined parallel resistance needs to be low enough that an insignificant amount of DC voltage is generated on the output on startup / shutdown.

Say you have a 2.2K load resistor on the output in parallel with a 300ohm headphone. On startup, the caps will charge, current will flow through the load and a DC voltage will develop transiently. Across the parallel resistance of the headphone and the 2.2K resistor (264ohm), any DC voltage will be low enough so as to be negligible, of no harm.

Now say you connect a 50K input impedance device to the output instead of a 300ohm headphone. In parallel with the same 2.2K load resistor, the 265uF caps will need to charge across 2.1K. The DC voltage at startup might not be negligible anymore, it might be close to 20VDC. If the input of the 50K device is not cap coupled itself, the input device could be exposed transiently to 20VDC. Will it survive? Depends on the device.

That's a long way of saying it would be better to use a separate output capacitor / load resistor combination for the pre-outs.

 

[mordy]

You could do it that way, but really not the best way. A power amplifier (or whatever gear is being fed by the pre-outs) will typically have an input impedance >50K, which means the output capacitance of the pre-outs does not need to be as large as it is for driving a headphone. For instance, a f-3dB point of 10Hz with a 50K load would only require an output capacitance of 0.32uF. This amplifier has an output capacitance of 265uF. Why does that matter?

On startup, these caps need to charge. On the output, they will do that through the parallel resistance of the load (headphone or amplifier fed by the pre-outs) and a parallel resistor. This combined parallel resistance needs to be low enough that an insignificant amount of DC voltage is generated on the output on startup / shutdown.

Say you have a 2.2K load resistor on the output in parallel with a 300ohm headphone. On startup, the caps will charge, current will flow through the load and a DC voltage will develop transiently. Across the parallel resistance of the headphone and the 2.2K resistor (264ohm), any DC voltage will be low enough so as to be negligible, of no harm.

Now say you connect a 50K input impedance device to the output instead of a 300ohm headphone. In parallel with the same 2.2K load resistor, the 265uF caps will need to charge across 2.1K. The DC voltage at startup might not be negligible anymore, it might be close to 20VDC. If the input of the 50K device is not cap coupled itself, the input device could be exposed transiently to 20VDC. Will it survive? Depends on the device.

That's a long way of saying it would be better to use a separate output capacitor / load resistor combination for the pre-outs.

Is it complicated to add an output capacitor / load resistor combination for the pre-outs?
The three tube headphone amps I have all work well as preamps. Two have preamp amp out and the third has separate hi/lo impedance headphone out. The designer told me to use the low impedance output when using it as a preamp.
However, the preamp out goes through a ss preamp which is used as a tone and volume control for the ss power amp. By adjusting both the volume on the tube headphone amp and the volume control on the ss preamp I can achieve a low noise floor. My impression is that this does not detract from the sound; occasionally I will need a slight tone control adjustment, depending on the tubes used, but usually everything stays neutral.

 

[L0rdGwyn]

Is it complicated to add an output capacitor / load resistor combination for the pre-outs?
The three tube headphone amps I have all work well as preamps. Two have preamp amp out and the third has separate hi/lo impedance headphone out. The designer told me to use the low impedance output when using it as a preamp.

It isn't complicated from a technical standpoint, but not something I am likely to do at the moment since I am on the verge of ordering the prototype chassis. If I were to do it, I would likely add a switch to change from headphone to pre-out, which means changing the layout, adding machined holes for the switch, RCAs, etc. Like I said, maybe I will consider it in the future, sorry to disappoint.

 

[mordy]

It isn't complicated from a technical standpoint, but not something I am likely to do at the moment since I am on the verge of ordering the prototype chassis. If I were to do it, I would likely add a switch to change from headphone to pre-out, which means changing the layout, adding machined holes for the switch, RCAs, etc. Like I said, maybe I will consider it in the future, sorry to disappoint.

The amps I have with built-in preamp out do not have a switch - both are on all the time. I can use either headphones or speakers, or both at the same time. Glenn liked to use both headphones and speakers at the same time for better sound.

 

[L0rdGwyn]

The amps I have with built-in preamp out do not have a switch - both are on all the time. I can use either headphones or speakers, or both at the same time. Glenn liked to use both headphones and speakers at the same time for better sound.

I can't speak to how it was set up in Glenn's OTL. I believe one of the other amps you are referring to is a Feliks Audio amp, they list a "preamp safety circuit" in their feature list, likely for the reasons I listed above, to protect against DC transients given that the two outputs are in parallel, the reason both are on at the same time. It isn't something I am interested in incorporating into this amplifier, but sounds like what you have is well-suited for your listening needs.