Hello everyone, I have been a little confused about the coupling caps with the .68UF bypass caps. I thought I'd find the coupling caps between the tube stages but it seems to be on the output, on the way to the phono jack. Am I looking at it wrong? I have tried some substitutions for the bypass caps and found that going somewhere closer to 1UF seems to avoid passing some of the harsher treble but retains the high end sparkle. I am hoping for some schooling in this regard. My dear old dad would have been a wealth of knowledge here and kept me on track; he was a great EE with a passion for old radio stuff, but he is long gone, and I miss the readily available expertise. He had me up to speed, 20 years ago, but I have just recently gotten back into electronics and have forgotten a lot. Hoping for some insight. Thanks!
The schematic is difficult to read without the tubes actually properly drawn on it.
But C8 and C9 are paralleled output coupling capacitors, and appear to be included within
an overall negative feedback loop. They block the high DC voltage present at the tube pin.
The nfb effectively increases their values by the amount of excess gain in the circuit.
You can use whatever bypass capacitor you want, since there's no general concensus
on what the best way is, if any.
But C8 and C9 are paralleled output coupling capacitors, and appear to be included within
an overall negative feedback loop. They block the high DC voltage present at the tube pin.
The nfb effectively increases their values by the amount of excess gain in the circuit.
You can use whatever bypass capacitor you want, since there's no general concensus
on what the best way is, if any.
Last edited:
Thanks, I agree the tubes should be drawn to look like tubes. I wish I could find a better schematic. It's for the Little Dot MKII headphone amp.
Am I right in thinking that the bypass cap is a good place to fine tune the mid and high output to taste by seeking roll off just above undesired frequencies?
`Who knows, you have to just try it. Some think it makes no difference, some do.
Parts located inside the nfb loop will not act the same as parts outside it.
If you want a filter, it's better to just add one that is designed for the purpose,
probably at the input.
Parts located inside the nfb loop will not act the same as parts outside it.
If you want a filter, it's better to just add one that is designed for the purpose,
probably at the input.
It seems to be working, so far. I like the idea of fine tuning the circuit to get the voicing I'm after. It sounds a little 2D and murky without the bypass caps, and it has more clarity and depth with even a .033uf in place. Will try .1 next and see if it lowers the threshold of highs.
The capacitor blocks DC portion of the signal and allows the AC signal through. This means one side you may have 100Vdc and the other is 0Vdc, the tube DC output has an AC signal superimposed on it so one plate of the capacitor gets a the DC+AC charge oscillating (current charging and draining quickly). This plate induces an apposing charge on the opposite plate, with the diametric between isolating the two plates so no current flows across it.
This, in layman's terms, is why you see caps changing phase (+ on one plate, attract electrons to cause - charge on the other thus current flow).
Caps also have harmonics but we'll ignore that and linearity of charge/discharge... but moving quickly on..
So in the design the coupling cap isolates one stage from the other and thus isolates the headphones from the 100+V on the tube plate.
The size of the plates (capacitance) is related to the maximum charge thus the current that can be supported. Next is that the resistance of the capacitor affecting the charge/discharge rate which affects the frequency supported.
Low frequencies require more charge capacity as the cap charges on one side of the sine wave, and discharges on the other. So more resistance slows this thus ESR (series resistance of the cap) defines the frequency it can couple between the plates.
The caps type also impact the speed of the cap. Eletrolytics are large capacitance but slower (although some are pretty quick with Al organics for example) but Polypropylene (PP) film caps are faster still. An electrolytic may have a V/uSec in the 100V the PP cap can have a V/uSec in the 10,000V. Either audio only needs about 5V/usec, at least for headphones. As most people want square waves then the component frequencies and transients require a higher frequency support.
Small caps such as the 220uF have higher ESR (in cheap caps anyway), so the 0.68uF is typically a PP type to provide high frequency support.
The HPA close to me has over 2600uF per channel but it's driving 55ohm headphones, around the MUSE ES it has 0.22uF FKP1 caps. It should also be noted that it's not a one way process. The tube sees the cap and the load beyond, so there's a design aspect to having higher capacitance and the ability of the tube to support that without being dragged down. In this case I have multiple parallel output triodes.
^^ Im not an expert but this is how I understand it.
Last edited:
Bear in mind that the 220uF output coupling capacitor is within the overall negative feedback loop,
so its low frequency pole is lowered by the amount of feedback, which will vary with the load and
the gain switching.
so its low frequency pole is lowered by the amount of feedback, which will vary with the load and
the gain switching.
Hi, Thanks. I have this one, but I believe it contains mods that its author had done. Also, this might be a really dumb question, but why does it only show one preamp tube but the two output tubes? Why didn't they just draw it complete, or is it because it is two parallel amps, but if that's the case, why show two output tubes? I know Im overlooking something here. Thanks again for that schematic.
One entire channel is shown.
This appears to be a White cathode follower output stage, which has two stacked tubes.
There is no output transformer. It only needs one tube for the voltage gain.
This appears to be a White cathode follower output stage, which has two stacked tubes.
There is no output transformer. It only needs one tube for the voltage gain.
It seems to be two parallel channels in the real world amp. either one works independently, as in, you can pull tubes from one side and the other is unaffected, except for shared heater current, etc.
Ha! That's my schematic from my thread rebuilding the Little Dot MKII over at Head-Fi. The only thing missing is the jumper to change from EF95 to EF91. I tried a bunch of bypass caps, and to be honest, I couldn't tell much of a difference. It's probably my stone ears. I had a pair of 4.7uF Mundorfs so I stuck them in there. The electrolytics were also upgraded to low ESR Nichicons. One concerning thing that I found out was that if one powered up the amp with high impedance headphones plugged in, there can be significant DC across the headphone as the 220uF cap charge up. In my rebuild, I used a switched headphone jack with 330 ohm resistors to take the DC, and once the caps charge up after a few seconds, then I plug in the headphones.
I enjoyed reading your notes on this amp! I'm having a lot of fun with mine! I put some Mullards in the front end and some NOS Soviet gold grid tubes in the output and it sounds really nice. Im waiting on some 470uf Nichicons to show up and will see if that gives me the little bit of sub bass extension I feel is absent. Good idea on the switched jack since the stock one is kind of crummy, anyway.
I'm experimenting with the bypass caps as a way to dip the mids around 2-4k as that area seems a but harsh with the stock ones, and murky without them. Im looking to get the clarity without any harshness. So far I think I can hear it but it might be placebo effect.
I know this looks a bit strange, but this sounds amazing, after a lot of experimenting. I wound up using orange drops because I already had some, and I have used them for years in my higher power tube amps with very pleasant results. I like that they are real foil, thus the large size, vs metalized film. I replaced the output bypass caps which were .68uF with .033uF and that seemed to remove some of the harshness in the sound. I think this raised the bypass frequency above the harsh high mids, but a little too much. I added a .1uF, to each side, in series with the .033uF to arrive at .024uF which seemed to lower the cutoff point into a nice musical range. Then, I also replaced the tube stage coupling caps from .22uF with .33uF. I was hoping to find a slightly lower rolloff point in the sub bass. It seemed to do it. I also tried replacing the output coupling caps with 470uF, but it made the amp feel sluggish and like it was working way too hard, so went back to the 220uF stock ones. I think the amp is the most musical its been at this point. I have tried a lot of different values, and this seemed to jump out as being a joy to listen to, as well as more forgiving on bad sounding recordings. Something about using the orange drops in series seems to sound different than arriving at similar values with one cap. Is there an explanation for this, or am I imagining it? I know there's a degree of placebo effect with anything you do in audio, but there also are many real world effects that small changes make. Has anyone experimented similarly with caps and found interesting results?
Last edited:
Yes. Capacitor effects are complex. Different brands and models of Foil and metalled film caps have different impedances. How much those differences impact the sound differs depending on the design and location. Temperature also plays a part in variation.
It's not just the cap itself but a change impacts the previous and following stage plus the overall amp if feedback is present. Audio is not omni directional so the previous stage find it harder to drive through a larger capacitor or the following stage may find a larger signal. Phase shifts may change and alter the stability of the amp if it's borderline.
Charge creates current hence how much current is required depends on the input impedance of the next stage, input capacitance of a following tube (parallel tubes are a good example of this). Swapping out a larger cap means more charge capacity is available hence more current flow - this can be harder for the previous stage to drive, and overload sensitive/high impedance input devices. Typically it adds low frequency extension but if the caps are large the previous stage can find it hard and thus you'll start hearing distortion impacting treble.
It's worth reverse engineering the circuit to understand what the capacitance should be and if an increase in value is really going to benefit it. Often voltage rating increases don't harm it and some will have better characteristics.
I'm listening to a recapped amp - previously with 'lytics in signal path. Now that is pure PP and MPP. Originally I used as Panasonic MPP 0.22uF which is mid rich but a switch to WIMA 1uF MKP10 + 0.22uF FKP1 (630Vac caps) coupling between the pre stage and the power stage has both really improved bass extension (bode plot shows this) and the highs are clearer by a large margin.
It's not just the cap itself but a change impacts the previous and following stage plus the overall amp if feedback is present. Audio is not omni directional so the previous stage find it harder to drive through a larger capacitor or the following stage may find a larger signal. Phase shifts may change and alter the stability of the amp if it's borderline.
Charge creates current hence how much current is required depends on the input impedance of the next stage, input capacitance of a following tube (parallel tubes are a good example of this). Swapping out a larger cap means more charge capacity is available hence more current flow - this can be harder for the previous stage to drive, and overload sensitive/high impedance input devices. Typically it adds low frequency extension but if the caps are large the previous stage can find it hard and thus you'll start hearing distortion impacting treble.
It's worth reverse engineering the circuit to understand what the capacitance should be and if an increase in value is really going to benefit it. Often voltage rating increases don't harm it and some will have better characteristics.
I'm listening to a recapped amp - previously with 'lytics in signal path. Now that is pure PP and MPP. Originally I used as Panasonic MPP 0.22uF which is mid rich but a switch to WIMA 1uF MKP10 + 0.22uF FKP1 (630Vac caps) coupling between the pre stage and the power stage has both really improved bass extension (bode plot shows this) and the highs are clearer by a large margin.