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Old 21st January 2013, 02:05 AM   #1121
6L6 is offline 6L6  United States
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F5 Turbo Builders Thread
Haven't they said that before...

Still, fantastic news!!
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Old 21st January 2013, 05:43 PM   #1122
ittien is offline ittien  Viet Nam
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Dear All,

I built the F5T V2 with 500VA Transformer for both channel. Bias at 350mV/1R resistor. all are working fine and temperature on heat sink about 50C. but for listening i feel missing detail at HF and the bandwidth is narrow. removed diode(because see it could be China made) but the sound and bandwidth also the same, dynamic range not good.

Checking voltage on each 1R resistor. and see it about ~20mV difference on each power FET. is that normal?

Could any reason that make F5T V2 not good in HF and dynamic range?
Thanks.
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Old 22nd January 2013, 02:23 AM   #1123
omishra is offline omishra  India
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Quote:
Originally Posted by ittien View Post
Dear All,

I built the F5T V2 with 500VA Transformer for both channel. Bias at 350mV/1R resistor. all are working fine and temperature on heat sink about 50C. but for listening i feel missing detail at HF and the bandwidth is narrow. removed diode(because see it could be China made) but the sound and bandwidth also the same, dynamic range not good.

Checking voltage on each 1R resistor. and see it about ~20mV difference on each power FET. is that normal?

Could any reason that make F5T V2 not good in HF and dynamic range?
Thanks.
What MOSFETs you are using - IRF or Fairchild?
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Old 22nd January 2013, 02:25 AM   #1124
6L6 is offline 6L6  United States
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ittien - Do you have genuine Jfet? Remove and measure Idss.
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Old 22nd January 2013, 02:48 AM   #1125
ittien is offline ittien  Viet Nam
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Originally Posted by omishra View Post
What MOSFETs you are using - IRF or Fairchild?
I used match quad Fairchild from Buzzford

@6L6: I bought Jfet from Japan with pre-matched. don't know it's genuine or not but it good. before I can not bias with fake Jfet for F5T. but with this JFet the bias setting easily and stable. i tried both BL and V type, result the same.
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Old 22nd January 2013, 04:31 AM   #1126
oiphy is offline oiphy  Norway
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Quote:
Originally Posted by ittien View Post
I used match quad Fairchild from Buzzford

@6L6: I bought Jfet from Japan with pre-matched. don't know it's genuine or not but it good. before I can not bias with fake Jfet for F5T. but with this JFet the bias setting easily and stable. i tried both BL and V type, result the same.
what railvolt? pics?
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Old 22nd January 2013, 05:46 AM   #1127
sangram is offline sangram  India
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20mV over 350mV is better than 6% matching. I would not be unhappy with that given that 4 1% resistors give a worst case matching of 4%.

The compression of sound that you hear is unusual. It is also not because of the diodes, those don't seem to be actually coming into play for most people.

IMHO 500VA seems to be on the adequate side for a single channel though, if it is for both you are slightly underpowered.
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Old 22nd January 2013, 06:12 AM   #1128
ittien is offline ittien  Viet Nam
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Thanks for your reply
@oiphy: I'm running with 32VDC rail. and after 2 channel running is 30.3VDC. I will take some picture later.
@sangram: I also though because underpowered, because all the component i used is good enough, i think. can you tell me what's "sound" problem if the transformer underpowered? may it cause lost detail at Hi Frequency? I'm hearing F5T without any pre-amplifier.
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Old 22nd January 2013, 09:28 AM   #1129
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by sangram View Post
........................given that 4 1% resistors give a worst case matching of 4%.
...................
No, it should read:
given that 4 +-1% resistors give a worst case matching of 2%.

But, did he use 1% or 5% or 10% resistors? Did he match the resistors? Did he even measure the resistors?
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Old 22nd January 2013, 09:49 AM   #1130
buzzforb is offline buzzforb  United States
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It i spossible that swapping there positioning will change the amount of variation you see. The P channel seem to be more sensitive to heat variation than th eN channel. I would guess that the P channel device ais where you are seeing you differences. Whil ei woul dlik eto have everyones fets show no difference in bias, there i snow way to match them for such baried conditions. Others may have input otherwise, but i doubt you are hearing a difference of 20mV in the output. Lacking HF is a bandwidth issue an dis more a result of current, capacitance and gain. I hav elsitened to mine for 3-4 months and cannot agree with your findings on the sound. I listen to classical music and have yet to find any dynami ccompression. The offset on my P channel fets is about 12mV. N channel are within 3mV.
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