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Old 12th December 2010, 04:52 PM   #11
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Nelson Pass View Post
A couple of thoughts,

1 490 mA is light on bias and will slightly alter the bass
.......



considering this :

Quote:
Originally Posted by kurtbauer View Post
........

Nelson, yes, that's ~230mV across each 0.47Ω resistor.
.........

K

it seems that overall bias is (0,23/0,47)x 4 = 1,96A

considering that F4 "by the book" have approx. same bias per pair ....

Quote:
Notes:
Output device bias is approximately .53 amps per device, which measures 0.25
volts across the .47 ohm source resistors.
dunno ..... ZM naively thinks that additional (20mV/0R47)x 4 , will not bring Nirvana ...

so - it seems that - either OP made some systematic mistake in both channels and loose bass control ...... or amps are just fine - just like you said , and he prefer fluffy tube bass
(not saying that tube amps have fluffy bass , just that his tube amp possibly have more fluffy bass than F4 )
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Old 12th December 2010, 05:42 PM   #12
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Thanks for the thoughts guys. Much appreciated!

ZM I'm with you in thinking I must have made some sort of mistake in the driver section. Where the current values for R3 and R4 within range? Are there other references that I could check? I'll also do a visual check of all of the components of course.

If I haven't made a mistake ( fingers crossed that i have!) and bias can indeed deliver nirvana, I would have to find a way to dissipate the heat since the current chassis max'd. That makes me wonder if I should consider a BA-2.

Very interested in answers to ZM's questions.

Thanks again guys!

K
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Old 12th December 2010, 07:50 PM   #13
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I really can't see what's wrong

everything seems fine ;

even in case that you are lacking ( and you're not ! ) in current through input Jfet buffer - first you'll suffer in highs , not bass department ....

try proposed test with your decent tube amp , as sort of source you already know , then report here
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Old 13th December 2010, 03:58 PM   #14
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Will do, ZM. Thanks.
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Old 13th December 2010, 05:48 PM   #15
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Kurt, I have the same situation with my F-4's but I am quite sure that it is because I calc'ed my bass box to be highly damped and then used a 15" woofer that was very highly damped also. I ended up with everything sounding fantastic, but the opposite of boomy bass: incredibly defined, but not as much, not as present...

Trouble is I have gotten used to it and might suffer with less (dare I say it) tight bass..
but I am planning to experiment with other cabinet options. Someday soon...

Just interesting that we seem to have the same situation but maybe for different reasons.
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Old 13th December 2010, 10:17 PM   #16
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I'm glad you weighed in, Variac. I was curious about your setup.

What you describe sounds vaguely familiar. Have you tried your F4 with other speakers? What preamp are you using? You did a standard 3-deep F4 right? What bias are you able to run? While I'm at it, what is your PS like?

I am trying to get some time this week for a deep dive on the driver boards to see if I've made any mistakes in construction. Then want to try with Bill's 6V6 with load resistors this time. Any maybe get out some fans and see what happens if I can managed the heat produced by deeper bias.
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Old 14th December 2010, 01:11 AM   #17
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Hey Kurt,

As always, its a mistake to assume that I have great sophistication about things ampish. I'm more of a typical DIYer wandering in the woods, hoping for good results and getting lucky usually, thanks to the great designs that we have available from our advanced members!

However, to complicate things, my power supplies ( I have two 2 ch F-4's) aren't stock.

For each 2 ch amp I have one supply:
My transformers are supposedly 1KVA each at +-37v, although they don't seem quite that big physically. . I run them with the primaries set for 220v so they put out about +-18.5v,using 110v and I guess 500VA but with the more robust construction of a somewhat bigger transformer.

I have C-L-C on each rail. Each cap is 30,000uF, 63v. My inductors are I think about 5 mH or maybe more. They are coathanger thick copper wire on toroids and are each about the size of a 400 VA transformer.

So I have 30,000uF-inductor-30,000uF on each rail with a 3.5uF and a .1uF film cap in parallel with the final cap.

I have the stock 3+3 MOSFETS, 6 total each channel, and they are biased stock.


My speakers are Basszilla's BUT with a highly damped JBL 15" bass driver that was used in speakers sold in Asia, not the one Dick Olsher recommends. It sounded a bit thin so I messed with the crossover a bit.. and that helped a lot, but still, not a ton o' bass.

At Burning Amp One I used an amp by Wrenchone I think for bass, which I believe he said had almost no damping factor. Much bassier. Otherwise I don't have that much experience with lots of other gear. I biamped with a chip amp on my bass driver for a while and it was about the same, but a bit more robust I think. I'm now using SY's Impasse pre and it's great, but bass wise no real difference than running the amps straight out of my CD player-except a hell of a lot louder and more authoritative of course! While the Impasse is a tube pre, it doesn't have soft uncontrolled bass - SY insists on linear, low distortion designs..

So it seems that the F-4 has a better grip on the bass drivers than you might think based on typical Pass Labs damping Factor numbers. But Nelson has told me that he doesn't see a good correlation between DF and bass in his experience.

Certainly perceived DF is better than many tube amps. But again, I haven't played around with it much. It sure is to die for on the mids and treble, and I think that if I got different woofers and/or revised my box, would get a bassier sound.. However the bass definition is awesome...


Mark

Last edited by Variac; 15th December 2010 at 06:57 PM.
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Old 16th December 2010, 09:24 PM   #18
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Default Testing Update

Hi again!

First of all, thanks for the rundown Variac. That's that's very helpful to know.

Bill and I did some testing last night over at his place with the 6V6 running into the F4'. This time we had an 11Ω load resistor across each 6V6 output. There was no motorboating this time.

There was also no bass. The positive characteristics of the F4' were still present but the bass was weak. Also, the sound had an overall distorted character.

We ran it again for a while with the pre directly into the F4' and concluded that we liked the sound of this setup better.

I'd love to hear what you guys think I might want try next but I was thinking of trying to get another few mV's into the bias without cooking anything. Then I was thinking I might try the 33,000 F cans that were originally in the two BA chassis across final C in the CRC PS chain for each rail. If you guys think that's a waste of time, please let me know.

Any and ideas welcome.

Cheers!

Kurt
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Old 16th December 2010, 09:28 PM   #19
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before messing with additional caps in PSU ( hardly that will help) , try to lower F4's DF ;

put few ohms in series with speaker output - to mimic highish DF of tube amp

try few values of these resistors
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Old 16th December 2010, 09:42 PM   #20
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Oh! That's what Nelson was talking about. Now I understand it.

Will do. More to come.

Thanks, ZM!
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