Mezmerize DCB1 Building Thread

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The ones you already use are not matched at all? Just random?

P.S. Try using some tamer LEDS for the 10.61V section in the group of five so to bring it a bit nearer 10V. That should help you finally judge your now offset better.
 
1) Which means if the Fets aren't matched :

1) you will get a very high DC offset and that will blow the output capacitor. Is this correct?

Or :

2) you will get a higher than normal DC offset, overwork the output capacitor (thus shortening its lifespan) and maybe the THD would be a little higher too.

But the sound quality remains the same.

Thanks.
PMChoong


To have excessive DC offset that will warrant an output capacitor coupler and a bit more THD maybe. Depends on chancing to the constant current fet over driving the signal fet above it.
 
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This is optimally a full DC coupled buffer. If it will show enough output offset it will need be blocked with a capacitor. That is all. Capacitors are to block DC up to their rating printed on them. They are not in danger doing that, its one of their normal jobs.
 
Hello Salas,
I have seen on “Toshiba 2SK170BL for sales” some DIYs requesting from Spencer 8.5mA matched 2SK170, or even matched to 9mA Idss.
Is there a particular reason for this, would JFETs with higher Idss at 0Vgs have better linearity, less distortions?

One more thing to ask, if I would like to have a more safe approach and add DC blocking cap on the output, and the input resistance of my Power Amp is 20kOhms, what cap value would be OK, can you also recommend a good cap chemistry-P/Number.
 
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At 9mA there is 90mW to dissipate for each Jfet as we use it here and better don't go higher for long term reliability, there is enough transconductance already. THD differences from say 7-9mA are not drastic.
Use 6.8uF I would say. Prices and subjective evaluations vary widely but an Obbligato MKP oiler, the one in black can with flying leads should be economical and effective since you need some uF.
 
Salas, thanks so much for your quick reply. Few more things to ask.
Also higher value, like 10uF should also work? Voltage, any voltage should work, 400V for example? Would 400V Jantzen metallized polypropylene be adequate choise? The conection point, is is OK to:
1. Connect cap between the 220 Ohm resistor (and the relay flying contact? In other words cut the trace between 220 Ohm resistor right lead on schematic (left one is connected to midle point of two JFETs) and insert it between relay. In this case the right side of the cap goes to the output through relay contact and the is connected to GND through existing 1M resistor?
2. Or to connect/insert a cap between the 1 MOhm resistor and output connector, adding another 1M resistor to the GND on connector side, the output to the power amplifier?
 
Just one more thought: for item (2), would this be even better to use a cap before relay. Like JFETs middle point goes to existing 220 Ohm resistor and cap in-series with it, from the right side of the cap: to 1M resistor to GND and to center (flying) relay contact, other relay contact goes to power amp input and through 1M resistor to GND (or even no this last 1M resistor at all relay contact straight to power amp?. Would this be more logical?
 
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It will be equivalent to having the cap in series with the existing DC coupled output on the PCB. Save hacking the board. Use 2 parallel RCA outputs, one direct, the other with the 6.8uF to 10uF output capacitor, so to have both DC coupled and AC coupled outlets. Depending on what you drive (AC/DC input coupled amps or DC input coupled amp with output DC fault protection relays) it will be a flexible configuration.
 
Finally I did the hot-rod first stage and I'm very impressed, the bass are incredible, as well the sound stage are wider and deeper.

I'm using two 16 Ohm resistors in parallel, the DCB1 are now close to 200m to 210mA (The voltage across the resistor drops a bit) (The DCB1 is cold).

A photos of the "near to finish" Mezmerize:

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5409104300_09a86ae524_b.jpg

5409106146_026e966deb_z.jpg

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Thanks Salas!

Hi all,

I'm looking for ideas for mounting the volume and source selectors on a thick front plate (the one from the store). It looks like I'll need to mount them similar to the one above but I can't see it in the photo. Any help would be appreciated!

Mark
 
An easy, easy extension shaft for the pot is a short piece of rubber tubing (automotive, vacuum or gas line) slipped over the pot shaft with another metal shaft on the other end. You can use a grommet or plastic tube (straw) where it penetrates the faceplate so it's not metal on metal. The knob will hide the penetration grommet. Hobby shops, and hardware stores have all different sizes of metal tubing. Doesn't have to be solid. Set screws on the knob will hold just fine.

[]--==----

pot >shaft>tubing>shaft

These ascii drawings don't usually work out well, but I tried.
Ron
 
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What is the wise target of Idss for matched 2SK170BL in order to get the lowest THD?
Do they have to be matched with +/- 0.05mA, or 0.1-0.15mA range would be adequate?
Just want to understand what numbers to get to Spencer ordering JFETs.

Its primarily about offset. THD here has to do with transconductance and signal level mainly. Try to get the best but 0.1mA deviation is no party stopper.
 
Salas, can you please clarify:
but an Obbligato MKP oiler, the one in black can with flying leads should be economical and effective since you need some uF.
It is a radial type of can, both flying leads on the same side (end), not the axial type of cap?
About the potentiometer-mutistep switch: what DIY-selfers use the most as a good aesthetitac approach and most nice rotation feeling, is it a Alps type 20kOhm potentiometer or DACT, GOLDPOINT Stepped Attenuator?