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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 17th November 2017, 04:09 PM   #121
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
OSCOns are nice for feedback shunt cap or for output coupling cap where low ESR gives better bass authority and minor leakage is not an issue. They are also nice for space constrained applications where dense volume is needed. I use them as output caps on my pocket class A amp because it lets me fit 2000uF in a tiny space with two units.
I agree that they are not needed for the 100uF 50v voltage reference filter caps here. Almost any through hole 100uF 50v cap here will work well and save quite a bit of money ($0.15 vs $1.75 for example). Through holes can be mounted on opposite side of SMT side to allow more flexible arrangements as I have done.
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Old 17th November 2017, 04:27 PM   #122
JPS64 is offline JPS64  Germany
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@xrk971
Why R10 carbon? or non inductive? Carbon as SMD resistor is very difficult to order!
MELF or MINIMELF enabling possibility to route signals under themself (starting standard size 1206) and IMHO they are a good choice for audio (from 0102(MMU) to 0207(MMB); SMD holder only for 0204(MMA)); also available from all distributors. Footprints are IPC compliant.
Remember, the metal core PCB doesnt allow vias.

@sottomano

IŽll check datasheets. Unfortunately, ELNA SILMIC SMD RVF (up to 10”) or RVO/RVM types not yet available as small quantities order. Will check PANASONIC, RUBYCON and others.

JP
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Old 17th November 2017, 04:42 PM   #123
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
You missed the revelation a page back. This was killer to find - almost had to swap out every part on board to duplicate the veroboard before I found it.

Here is FFT with metal film SMT:
Click the image to open in full size.

Here is carbon on R10 feedback resistor:
Click the image to open in full size.

This confirms anecdotal reports that metal thin film resistors, when used in audio path, "sound harsh". It is extra sensitive in the feedback position. Matched LTP transistors hardly made any difference, and the difference it made was to reduce H2 even farther relative to H3.

You can take an axial carbon resistor and bend the legs to mount on 0812 SMT pad easy enough. Luckily, it is just this one resistor that needs to be carbon.
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Last edited by xrk971; 17th November 2017 at 04:45 PM.
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Old 17th November 2017, 04:48 PM   #124
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output-layout-top-png

Can you please show silk layer to see component labels and outlines? I only see 5 actives?
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Old 17th November 2017, 05:08 PM   #125
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
There is one other trick this circuit can do if you make R15=3.3R and R14=2.2R and drop Vcc to 24v. You get a pretty nice headphone amplifier! Here it is driving 4vpp into 250ohm load:

Click the image to open in full size.

This is for 72mA bias current and about 900mW dissipation in each of the output devices and CCS MOSFET (still within spec).

Also, change output cap to bigger value if using lower impedance headphones. I put in 2200uF in sim but probably 470uF is enough for 250 ohm phones.
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Last edited by xrk971; 17th November 2017 at 05:30 PM.
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Old 17th November 2017, 05:30 PM   #126
Sk8Ter is offline Sk8Ter  United States
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Very Impressive X.....keep up the great work!
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Old 17th November 2017, 06:16 PM   #127
JPS64 is offline JPS64  Germany
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carbon 1/4W replaced.

100” --> PANASONIC FC (leakage 0.01CV), others stay OSCON.

All other parts (input/output caps, volume potentiometer, cap-multiplier, CRCRC) on mainboard (in progress).

JP
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File Type: png TOP_2.png (579.1 KB, 133 views)
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Old 17th November 2017, 06:39 PM   #128
Vrystaat is offline Vrystaat  South Africa
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XRK, do you think you might be able to replicate this with carbon resistors on some of your other amps ?
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Old 17th November 2017, 07:37 PM   #129
Sk8Ter is offline Sk8Ter  United States
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Default Not to Steel X fire here

I have been playing with different manufactured feedback resistors in my amplifier,they can make huge differences in sound for sure

I just did install some carbon films and you can clearly hear the third harmonic is lessened, this really can fool you into thinking you have less distortion for sure

I wonder how long for resistors to break in
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Old 17th November 2017, 08:14 PM   #130
Ultima Thule is offline Ultima Thule  Europe
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Post Resistor choice in the FB path

Quote:
Originally Posted by xrk971 View Post
This confirms anecdotal reports that metal thin film resistors, when used in audio path, "sound harsh". It is extra sensitive in the feedback position. Matched LTP transistors hardly made any difference, and the difference it made was to reduce H2 even farther relative to H3.
X that is no doubt a good observation and well worth to keep in mind, may we know exactly what brand, power rating and tempco the mentioned resistors were of, thanks.

Last edited by Ultima Thule; 17th November 2017 at 08:16 PM.
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