Recommendation for 5-10W amp.

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#183
VSSA uses MOSFETs at output as Source followers and BJTs as VAS which was Fab's idea years before VSSA and I used BJTs at the input of F5 also before VSSA so this amp is just a small, single rail, BJT+LAT version of F5...

I guess it fulfills the post #1 requirement - to sound different from ACA. 🙂
 
#187
Adding a volume ctrl.

Hi,

If I want to add a volume control, which value do I choose and is my volume located properly on my schematic ? I have chosen 20K but is this a good value, I haven't bought the pot yet. My source will be either a CD player or the output of a future DAC.

How do you choose between, 10K, 20K or 50K. Just want to make sure my chosen value is OK and won't affect the bandwidth and overall gain.

p.s. Pic of the amp being built.

Thanks,
Eric
 

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#188
Chassis design looks nice and sturdy. A metal bottom and top plates bolted to the heatsinks should help remove a bit more heat.
The volume control placement on the schematic looks ok to me. Considering your sources are CD player and DAC with usually lower than 1K output impedance, I would say your 20K choice is also acceptable. Juma however has more listening hours on this ckt type.
 
#189
e_fortier said:
...How do you choose between, 10K, 20K or 50K...
Click to expand...

We choose the pot value by two criteria:
- as high as possible to avoid overloading the previous stage
- as low as possible to avoid creating a strong low-pass filter that series pot resistance makes with the input capacitance of the next stage

Sensible compromise between these two contradicting demands is a 10k pot. Any previous stage that can't handle 10k load needs a buffer anyway because it won't be able to properly drive the cable capacitance either.
10k pot puts maximally 2k5 series resistance in the line which won't build very strong LP filter with typical input capacitance of pre/amp.

If you already have a good 20k pot it's OK to use it with this amp.

However, my experience is that amps with BJT input stage sound better with a buffer (B1 or similar) between the pot and the amp's input. You might come to different conclusion, since this amp's input BJTs work with low Ib (5-6uA). Anyway, it's easy to test it...
 
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#190
I generally buffer my volume potentiometer.

This allows me to use any value from 10k pot to 1M pot.

If you don't intend buffering the vol pot output then:
1.) you must make allowance for cable and receiver capacitance. The vol pot MUST be able to drive that capacitance.
2.) the maximum output impedance (Ro) of the vol pot will interact with the receiver input impedance (Rin). Aim for a ratio {Vol Pot Ro} : {Receiver Rin} < 1:10 and preferably <1:20

To assist with meeting those two conditional requirements, I suggest you adopt ~10k vol pot.
 
#191
10K it is then. Thanks to all for the explanation.

Question : If I was going to insert the B1 in there what JFET could I use instead of 2SK170, LSK170 or 2SK370...these seems to be very hard to get, is there any new component that are easier to buy (buying 2SK170 now is risky, I like genuine stuff)

Would I only need to add the 5 circled components to buffer my volume, the rest seems to be already in the amp ?

Eric
 

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