It may also imply that the noise is injexted before the gain circuitry. Perhaps in pot, perhaps elsewhere.Thanks for the feedback y'all
I have now listened to the preamp in my speaker setup for a couple of hours. The pre replaced a DCB1, over which the BA3 - to my ears - has notable improvements in bass slam and resolution. Very pleasant still, I believe thanks to 2nd harmonic dominance.
However, I'm encountering an interesting problem with hum(!), which I think somehow stems from the volume pot. I did not encounter this problem with the headphone setup. The source (dam1021) is completely free of hum, so I would excluded that as the culprit. The power amp (lpuhp) is a bit finicky concerning hum free operation, but the volume pot is at the input of the pre, thus isolated from the power amp through the preamp. I would therefore also exclude the power amp as the culprit.
The volume pot is a Khozmo 10k relay based attenuator, make before break, Amtrans AMRG resistors.
So with the volume at 0 (-60dB from input), there's zero hum. Turning up the volume to 30 (-30dB from input) there is a very slight increase of hum, just barely audible with the ear right next to the speaker (power amp gain at +15dB). With the volume going from 30 to 31 (1dB step), there's a significant increase in hum. There's a bunch of relays clicking in the Khozmo attenuator, so I suspect there's some impedance change going on. The hum further increases going up to 62 (-1dB from input). Going to 63 (0dB), the hum completely(!) disappears again.
Really strange behavior. Gonna investigate some more in the coming days.
You have completed an impressive build. But just to rule out a usual culprit: how is the star gnd connected to chassis? Is there a resistor or glb/thermistor between star and chassis? It is quite easy to identify whether that might be your issue. Just disconnect audio gnd from chassis gnd, and see if there is a difference.
You’ll solve this!
Regards,
Andy
I have use my preamp for many years now. So far I have had the attenuator after the BA-3 but I plan to get a new attenuator and put it before the BA-3 instead since some sources with high output levels overdrive the BA-3.
I will use a relay based stepped attenuator. What would be the best impedance? I have some examples:
1. 10K input impedance (seen by the source) and 0-13K output impedance (seen by BA-3)
2. 27K input impedance and 0-13K output impedance.
The source will need to drive the attenuator impedance in parallel to the the BA-3 impedance right? That means 10K||47K=8.25K for option 1 which feels a little low. For option 2 it will be 27K||47K=17.15K which feels much better. On the other hand, for option 2 the BA-3 will see a higher impedance which is not so good. What would you chose?
I will use a relay based stepped attenuator. What would be the best impedance? I have some examples:
1. 10K input impedance (seen by the source) and 0-13K output impedance (seen by BA-3)
2. 27K input impedance and 0-13K output impedance.
The source will need to drive the attenuator impedance in parallel to the the BA-3 impedance right? That means 10K||47K=8.25K for option 1 which feels a little low. For option 2 it will be 27K||47K=17.15K which feels much better. On the other hand, for option 2 the BA-3 will see a higher impedance which is not so good. What would you chose?
One quick question,
my busy season started so even tho I collected the Sigma 22 boards, the BA-3 one, the I-Select and a good part of the components I have to delay the build to fall when the season fades out.
Question being, the FW F4 finally made it home, it's an amazing build, it's huge but... there is a BIG BUT, the output level is low, 25W not enough for my JBL L220, I read my Audio Research SP9 delivers 21db to the output and the volume knob all the way to the max still feels "poor", would the extra gain of driving the BA-3 with 30+ volts give it some punch???
I am so disheartened, it took so long of a wait (and quite some money) to have the amp done and...
Grazie
my busy season started so even tho I collected the Sigma 22 boards, the BA-3 one, the I-Select and a good part of the components I have to delay the build to fall when the season fades out.
Question being, the FW F4 finally made it home, it's an amazing build, it's huge but... there is a BIG BUT, the output level is low, 25W not enough for my JBL L220, I read my Audio Research SP9 delivers 21db to the output and the volume knob all the way to the max still feels "poor", would the extra gain of driving the BA-3 with 30+ volts give it some punch???
I am so disheartened, it took so long of a wait (and quite some money) to have the amp done and...
Grazie
Source is a little Beresford Caiman SEG DAC, I will try with the direct CD output (a vintage modded Sony 505ES) to see if it is a little louder and also to change the input on the preamp, dunno if all the line inputs have the same gain but I seem to recall they all are the same besides the CD one...
With the tube amp (rated 45W) I never exceeded hour 11 on the volume knob, now at full power it can't seem to tickle the JBLs.
Which kind of measurements you suggest to perform?
Grazie, you all are so very kind and this amp is a beauty, will show you pics when this moment is behind the back... (damn Klipschorn, now I start to think about those again, if I only had two corners with no doors and a much bigger bank account)
Edit: listening directly off CD and it is what it is, low...
With the tube amp (rated 45W) I never exceeded hour 11 on the volume knob, now at full power it can't seem to tickle the JBLs.
Which kind of measurements you suggest to perform?
Grazie, you all are so very kind and this amp is a beauty, will show you pics when this moment is behind the back... (damn Klipschorn, now I start to think about those again, if I only had two corners with no doors and a much bigger bank account)
Edit: listening directly off CD and it is what it is, low...
The F4 has no gain. Needs to be fed by a preamp capable of swinging voltage.
"Virtually all active line stages achieve adequate gain and voltage swing to drive the above combination. Basically you need a linestage with about 12+ dB gain and 10+ volt maximum output."
https://www.firstwatt.com/pdf/prod_f4_man.pdf
"Virtually all active line stages achieve adequate gain and voltage swing to drive the above combination. Basically you need a linestage with about 12+ dB gain and 10+ volt maximum output."
https://www.firstwatt.com/pdf/prod_f4_man.pdf
I made a dumb test, picked my old Adcom GFP-345, play a tune and measure the db level with an iPhone app, average in the 90db, switched back to the SP9 and with the same tune it gets in the 80db, something wrong with the Audio Research?
I will try and mount again the tube amp with the SP9 as it was before the F4 got home and report back
p.s. to measure DAC output I use the same routine, play the 1Khz tune and measure DC output?
I will try and mount again the tube amp with the SP9 as it was before the F4 got home and report back
p.s. to measure DAC output I use the same routine, play the 1Khz tune and measure DC output?
Reading the owner's manual, there appears to be a 6dB cut option internal to the preamp.
Page 8. Requires soldering.
Wonder if someone exercised that option.
https://audioresearch.com/wp-content/uploads/2021/08/SP9_Manual.pdf
Page 8. Requires soldering.
Wonder if someone exercised that option.
https://audioresearch.com/wp-content/uploads/2021/08/SP9_Manual.pdf
Ok, now you gotta tell me how did you get there, an AR aficionado, you own one...? You guys never end to positively surprise me, I am amazed at knowledge and patience.
I switched from the SP9 to the GFP345 and this latter one has a little more gain, playing music from the laptop to the pre straight away as to eliminate a source of doubt, playing youtube and I am quite amazed at detail of this thing.
So if I get it right the SP9 is set by default with a 6db limiter on all the outputs besides the tape outs, have to remove the bottom plate and figure it out then ask a skilled friend to do it for me.
By the way, volume knob all the way to max but the thing doesn't heat up, is it because it is not swinging full power as of low output gain of the preamp?
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What doesn't heat up? The Preamp or the F4?
The F4 is a Class A amp. It should be running quite warm. Did the builder setup the bias and offset? Did they do a final test/check out? Listen to it to ensure it was working correctly?
the F4 is quite cool, yes, the person who assembled it performed all the tests, dunno if I am allowed to post a link but this is his FB page and there are videos and pics of the assembly and test numbers