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Old 31st October 2013, 05:15 PM   #1431
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No. This aspect of the circuit is in Nelson's build instructions.
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Old 31st October 2013, 11:24 PM   #1432
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Lots of progress to report.

The buzz problem is fixed. I was using a twisted pair wire of wires to connect the RCA inputs to the front end board. EMI turned out to be the culprit. I did not have any shielded cable around so I used some cat 6 cable and that works like a champ. No hum or buzz as long as I have a cable connected to the input.

If I short the inputs the amp is dead quiet. Thanks to 6L6 for setting me on that course - it told me something other than the amp was causing the issue.

The rustling sound in the right channel is another story. I am using an Apple TV box connected to a Schiit Bifrost DAC as my source. I am using the volume control on my iPhone via Airplay, as I have not built my passive volume control yet. I suspect what is happening is the digital volume control function is causing the noise. If I unplug the Apple TV or the DAC the rustling sound goes away. In any case, that's my theory and I will keep everyone posted. My plan is to build the PVC and see what happens.
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Old 13th November 2013, 04:11 AM   #1433
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Default BA-3 Gain stage ?

After looking over Nelsons super cool BA-3 gain stage the one without feedback I began to wonder if this or a similar modded design might have enough gain to be used as a stand alone very high gain preamp.

I was told by someone that the balanced version has bucketloads of gain, not being aware of some of the parts just guessing it seems like it might be good for about 20-25 db tops but this is a complete guess not knowing the value of q-3 for starters.

Would the gracious Mr Pass or someone else savvy with his designs be able to resolve my query?

What is the gain in db with 600ohm impedance at each end of the BA3 balanced gain stage for the power amp?

I desire to find a gain stage that can be used for a good 40 db and doubled up a good 70 db for things such as very low output moving coil, passive filter front /back ends and microphone preamps. It sure would be nice to adjust the gain via local feedback rather than be stuck at one level with attenuation padding or loop phase cancellation on a solid state design.
Obviously the gain adjustment would be far less linear in such a beast than that with a big negative loop but I can live with that.

If anyone has an answer and even if this is the wrong forum for such a query I apologize as this site is vast and robust and much better than TV so please point me in the right direction.


Thank you,

Mick
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Old 20th November 2013, 08:27 PM   #1434
pidesd is online now pidesd  Canada
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am i wrong if i would say this pre could make a nice current dac output stage, permitting to have a nice low value I/V resistor at the dac output if needed. One then could fiddle with the gain and I/V resistor value.

I'm just a bit worried about the noise that would such a gain induce. I'm not an expert though.

...or maybe change the first stage like in the picture

Last edited by pidesd; 20th November 2013 at 08:50 PM.
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Old 20th November 2013, 09:05 PM   #1435
pidesd is online now pidesd  Canada
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Default image

image
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File Type: jpg I_V 2.JPG (31.5 KB, 242 views)

Last edited by pidesd; 20th November 2013 at 09:13 PM.
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Old 20th November 2013, 09:43 PM   #1436
pidesd is online now pidesd  Canada
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mkjekyll,

i m not sure if the BA-3 would yield good performance numbers with a gain of 40dB, but maybe it could. I'm no expert though...

If youre looking for a good phono stage, definately look for Salas Phono. I dont know if it sounds good or suit your tastes but salas makes VERY good stuff in general.

Last edited by pidesd; 20th November 2013 at 09:46 PM.
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Old 23rd November 2013, 01:25 PM   #1437
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Here's an update on my BA-3 build.

I was chasing a buzzing issue that was driving me nuts. It would come and go. Every time I thought I had it fixed it would come back. Well, it turned out to be EMI from a dimmer switch. I finally realized one day that dimmer on = buzz. I believe it was radiated emissions being coupled into the amp by the interconnects. I was able to get the buzz down to tolerable levels by proper cable dress and implementing a proper star ground. This was with the AC safety ground tied to the star ground and chassis.

I then decided to isolate the star ground from chassis and safety ground using a diode bridge and CL60. The result was absolutely no buzz, with ear to the speakers, with the dimmer on or off. The amp is now absolutely dead quiet.

When I built the amp I could only get jfets with an idss of about 3.5 ma. I could not get Q3 and Q4 biased with the stock parts values so I substituted a 1K pot for P1 and P2 and changed R6 and R7 from 100 to 1000 ohms. This worked, but the bias of Q3 and Q4 was unstable even after warm-up.

I then realized from reading the article that R6 and R7 set the gain of the jfets along with R8. (Of course I could have picked that up from the schematic but I was blind to it at the time.) Instead of unity gain, I had a gain of about ten. Anyway, long story short, I bought some "c" grade Linear Systems parts, changed R6 and R7 back to 100 ohms and now all is well.

I am running the output stage at 500 ma bias per device.

Today I am going to jack up the bias of Q3 and Q4 to about 100 ma and see how that sounds.

Speaking of sound, the thing sounds pretty spectacular. The sound stage extends well beyond the speakers. I like to listen loud, and I have yet to have any listening fatigue. Plenty of bass into 92 db 4 ohm speakers. This thing is as smooth as Macallan 25.

My thanks to Nelson and the rest of you who designed the amp, laid out the circuit boards, designed the 5U chassis, found sources for parts, did the documentation and BOMs, maintained the threads, operated the store, shelled out the $$ for inventory, answered my silly questions, etc. It's obviously a labor of love for you folks and I am really enjoying the result.

I have a friend coming over today to give it a listen and I'm betting I will be building one for him next.
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Old 23rd November 2013, 01:36 PM   #1438
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Macallan 25 ...

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Old 23rd November 2013, 02:30 PM   #1439
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Quote:
Originally Posted by mstomaso View Post
I was chasing a buzzing issue that was driving me nuts... it turned out to be EMI from a dimmer switch.

I then decided to isolate the star ground from chassis and safety ground using a diode bridge and CL60. The result was absolutely no buzz, with ear to the speakers, with the dimmer on or off. The amp is now absolutely dead quiet.
Fantastic! It's always something fairly simple. This goes to show how important those few ohms provided by the thermistor are to lifting the PSU gnd potential and quieting down the circuit.

Quote:
I bought some "c" grade Linear Systems parts, changed R6 and R7 back to 100 ohms and now all is well.
Great news - and the lesson for the memory banks here is that the higher side of the 'BL' (LS '2') grade will also work well, say 8-10Ma or a bit more.


Quote:
Speaking of sound, the thing sounds pretty spectacular. ... This thing is as smooth as Macallan 25.
Is there any higher praise possible?


Quote:
I have a friend coming over today to give it a listen and I'm betting I will be building one for him next.
Remember to include him in the process - even if you just have him looking over your shoulder, he will learn something. And that's the whole point, right?

My most sincere and heartfelt congratulations on the success of your project.

Now let's see some photos!!
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Old 23rd November 2013, 02:33 PM   #1440
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by mstomaso View Post
....... I like to listen loud, and I have yet to have any listening fatigue. Plenty of bass into 92 db 4 ohm speakers.
I know I sound like a broken record (NPI) but what do you consider loud - and are you using a pre with gain to get you there? Can you get the SPL you want/like with front end adjustments only?
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Last edited by bcmbob; 23rd November 2013 at 02:38 PM.
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