I confirmed the X161 pinout for measurements and even connected the other board with the ribbon cable to verify readings at the (empty) daughter card locations.
It it normal to only have 1-2V adjustment range with the trimmers? Granted there's no load yet, but I suspect something is not quite right with my build.
I've checked installed component values against the schematic and everything seems correct. Parts are from Vunce's Mouser cart. Optional snubbers are installed - anyone else running those values?
Any test points I can measure to ensure correct function?
BK
You can skip the capacitors completely then and go DC coupled. If using only film caps, value will not be large enough to achieve sufficient bass extension.
Ah, well that's a relief and I'll call it done - thx! It's a crowded board and thick, so re-dos would have been tricky.
+/-25V is suitable for Melbournes. I'll build those today according to the Yarra-specific changes in post #1, but if there's other key info hiding in the threads please holler!
BK
GB Status Update
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arthur - 6 WBA18 boards+3 Yarra boards - Pre-order placed
afoor - 2 WBA18 boards - Pre-order placed
jhofland - 2 WBA18 boards - Pre-order placed
skunark - 4 boards, 1 yarra board - Pre-order placed
TungstenAudio - 2 WBA18 boards + 2 PCA - Pre-order placed
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windwardmt - 4 WBA18 boards + Yarra - Pre-order placed
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poseidonsvoice - 2 WBA18 - Pre-order placed
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bologna1 - 2 PCA - Pre-order placed
Urien - 2 WBA18 + 1 SLB - Pre-order placed
Roundtoit - 2 WBA18 boards - Pre-order placed
Meanie - 2 WBA18 - Pre-order placed
Kokanee - 2 WBA18 boards + 1 Yarra MB - Pre-order placed
Mike(need DIYA handle) - 2 WNA18 + 1 Yarra MB - Pre-prder placed
Carl Huff - 2 WBA18 boards
Big E - 2 WBA18 boards
Russellc - 2 WBA18 boards
Oracle1 - 2 WBA18 boards
ZUM911 - 2 WBA18 boards
Modman - 2 WBA18 boards
mordikai - 2 WBA18 boards + 2 Melbourne + 1 Yarra - pre-order placed
vvs07- 4 WBA18 boards + 2 Yarra board sets
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So I had planned for one of the Yarra builds to make the HPA Melbourne version with 15v trafos, but now I have both the PCA and WBA18 boards coming. The 15v trafos are in hand. Will they work for either of those to boards? I’ve been slammed at work so keeping up with this thread has been tough this summer.
The other build will be a straightforward preamp to drive my F5Tv2, plus what ever other amps I build like the m2x. With the Melbourne boards should I get 18v or 22v? Trafos?
Free time to start building is a month away and I want to be ready.
The other build will be a straightforward preamp to drive my F5Tv2, plus what ever other amps I build like the m2x. With the Melbourne boards should I get 18v or 22v? Trafos?
Free time to start building is a month away and I want to be ready.
X,
I looked over the preamp and was really pleased it is very simple and will sound wonderful because the topology is single ended.
One point: C101, the gate coupler for the jfet, is seeing a bootstrapped 47k bias resistor which will be around 1M in reality. So 4.7uF is not needed; you could use a small film cap of 100nF and have the same FR corners and this would be much cheaper.
Brilliant layout and unashamed, simple circuit.
Hugh
I looked over the preamp and was really pleased it is very simple and will sound wonderful because the topology is single ended.
One point: C101, the gate coupler for the jfet, is seeing a bootstrapped 47k bias resistor which will be around 1M in reality. So 4.7uF is not needed; you could use a small film cap of 100nF and have the same FR corners and this would be much cheaper.
Brilliant layout and unashamed, simple circuit.
Hugh
Hi Hugh,
Are you talking about the PCA add-on cars?
Note that Vcc needs to be under 19v (not 63v) correction has been made in Aksa Lender thread, I need to update this schematic:
Wouldn’t the input coupling cap see the 47k plus the circa 270ohm to ground? I actually specified 2.2uF there but JP doubled it. However, that cap is on the motherboard where people are free to roll caps and use big fat boutique oil filled ones etc. a 10uF aluminum in oil Mundorf EVO M cap fits there well.
Are you talking about the PCA add-on cars?
Note that Vcc needs to be under 19v (not 63v) correction has been made in Aksa Lender thread, I need to update this schematic:
Wouldn’t the input coupling cap see the 47k plus the circa 270ohm to ground? I actually specified 2.2uF there but JP doubled it. However, that cap is on the motherboard where people are free to roll caps and use big fat boutique oil filled ones etc. a 10uF aluminum in oil Mundorf EVO M cap fits there well.
Thanks for answer X. YES the add on car.
Since the both end of the 47k bias track the input voltage,
This maintains constant bias current and since the input
current is constant the signal sees a far higher impedance than
47k. 1M would be closer so you can reduce the capacitance
to reflect the reality.
It really is not intuitive but one of the big reason we use source biasing.
I really like this simple circuit.
HD
Since the both end of the 47k bias track the input voltage,
This maintains constant bias current and since the input
current is constant the signal sees a far higher impedance than
47k. 1M would be closer so you can reduce the capacitance
to reflect the reality.
It really is not intuitive but one of the big reason we use source biasing.
I really like this simple circuit.
HD
Hi Hugh,
You should like it - it’s your circuit from the PCA, just modified for the Yarra (and now Aksa Lender) card formats! I still like its sound a lot - spent over 3hrs today with the PCA and a pair of balanced armature IEMs on my commute. A fine sounding headphone amp. You really don’t need more than 2 transistors for great sound.
Thanks for the explanation of how it tracks the voltage - so the source basically follows the gate and hence the differential across the resistor is like a 1Mohm? Well I will have to try my super special secret 100nF SMT PEN film caps on this. They are reported to have a sublime sound. Not very expensive for a little 100nF 1210 50v SMT - but interesting that it’s a film cap and not MLCC for SMT. I bought a few years ago for an opportunity to try.
https://www.mouser.com/datasheet/2/40/cb-pen-776513.pdf
You should like it - it’s your circuit from the PCA, just modified for the Yarra (and now Aksa Lender) card formats! I still like its sound a lot - spent over 3hrs today with the PCA and a pair of balanced armature IEMs on my commute. A fine sounding headphone amp. You really don’t need more than 2 transistors for great sound.
Thanks for the explanation of how it tracks the voltage - so the source basically follows the gate and hence the differential across the resistor is like a 1Mohm? Well I will have to try my super special secret 100nF SMT PEN film caps on this. They are reported to have a sublime sound. Not very expensive for a little 100nF 1210 50v SMT - but interesting that it’s a film cap and not MLCC for SMT. I bought a few years ago for an opportunity to try.
https://www.mouser.com/datasheet/2/40/cb-pen-776513.pdf
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Doesn't the R105-R106 voltage divider, limit the effectiveness of the impedance bootstrapping? Using resistor values from the "2SK209" column of the table, the resistor junction sees about 80% of the source voltage (which itself is 99.5% of the input voltage). So the input impedance increase is about a factor of five (== 100/(100-80))?
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