Several years ago, before I started meddling with vacuum tubes, I bought a relatively popular Chinese OTL headphone amp, the Little Dot Mk II. It sounded pretty good. In my ham-fisted attempts to "improve" the sound, I swapped out capacitors and such without knowing their function in the circuit. Many lifted pads and bodged wires later, the amp got shelved away.
After seeing the 6AS7 amp that I'd just completed, my kid wants one too, but smaller in size. I remembered the Little Dot, took it off the shelf, cleaned off the layer of dust, and cracked it open. Because the PCB's so jacked-up, I thought I'd build a point-to-point clone using the power transformer. I traced out the circuit:
The front end is easy, a triode strapped pentode. The output section appears to be a White cathode follower. In Merlin's book, he has a voltage divider providing the bias for the upper triode. Here, it looks like the bias is achieved by the 120R resistor between the 2 sections. Are there any improvements that can be made in terms of safety of operation and sound quality?
Thank you.
After seeing the 6AS7 amp that I'd just completed, my kid wants one too, but smaller in size. I remembered the Little Dot, took it off the shelf, cleaned off the layer of dust, and cracked it open. Because the PCB's so jacked-up, I thought I'd build a point-to-point clone using the power transformer. I traced out the circuit:
The front end is easy, a triode strapped pentode. The output section appears to be a White cathode follower. In Merlin's book, he has a voltage divider providing the bias for the upper triode. Here, it looks like the bias is achieved by the 120R resistor between the 2 sections. Are there any improvements that can be made in terms of safety of operation and sound quality?
Thank you.
Are you sure the 470K resistor feeds to that side of the 10K resistor at the input?
The 2.2uF cap doesn't appear to serve any purpose.
The 2.2uF cap doesn't appear to serve any purpose.
Yep, that seemed somewhat screwy, but that is what's on the PCB.
I think the 2.2uF cap is there to keep whatever DC from the grid off the wiper of the pot.
I need bifocals. The NFB resistors are indeed on the tube side of the 10k grid stopper. Corrected schematics:
The input coupling capacitor is not necessary. If you really want to keep it,
move it to before the level control.
move it to before the level control.
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If I was building this for my own use, I'd definitely get rid of the input coupling capacitor. However, this will be for my son who will be going back to college in the fall should this current situation blows over. I'm not about to trust him or his friends to always plug in the right thing. (In more ways than one.)
I´m also tempted to go for a OTL headphone amplifier, but I don´t like the idea of having some voltage on my head... So I prefer to get some reasonable transformers.
If you are not going to move the 2.2uf input cap to the input as suggested above, I would add a 100K resistor from the grid of the 6AK5 to ground. This should reduce the grid leakage current from going through the headphone.
