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Old 26th March 2012, 06:04 AM   #111
reytnz is offline reytnz  New Zealand
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Thanks, I will try that one. Any reason for the value??
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Old 26th March 2012, 06:58 AM   #112
juma is offline juma  Serbia
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Quote:
Originally Posted by reytnz View Post
...Any reason for the value??
If you mean the proposed C4 value (1uF), it's determined by F5's input resistance (101k) and by our wishes:
- to avoid high-pass filter at amp's input by using too small a value ;
- not to use very big value because good caps are cheaper when smaller .
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Old 26th March 2012, 12:44 PM   #113
AndrewT is offline AndrewT  Scotland
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Hi,
I am about to try breadboarding this circuit.

I will try +0dB, +6dB and +12dB.
Do you expect any of these gain versions to require some or different compensation?
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Old 26th March 2012, 01:06 PM   #114
juma is offline juma  Serbia
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I tried 6dB and 12dB gain - no compensation was needed.
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Old 26th March 2012, 07:03 PM   #115
tp147 is offline tp147  Germany
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Hi Juma,

I'm looking for a suitable preamp for my new F3 and I like your design from post 53.

Currently I drive the F3 with my Naim NAC72 with its 22dB gain, not enough for the 12,5 dB from the F3.

How much gain has your design as posted and which gain is possible without disadvantages?

thanks & cheers

Thorsten
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Old 26th March 2012, 10:12 PM   #116
juma is offline juma  Serbia
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tp147,
you should consider more than just the gain - F3 puts out just 15W at 8 Ohms which translates into 11V RMS at 8 Ohms. F3 has gain of about 12dB (4V/V) and that means that it needs about 2.7V RMS at its input for full power.
If you say that NAC72 with 22 dB (12.6 V/V) gain can't drive F3 to full power it means that your source has less than 200mV output and that looks strange.
Think once more and analyze where's the problem:
1. low power (just 15W from F3)
2. low output from your source (less than 200mV)
3. low output level of your preamp (can't deliver 2.7V RMS into 10k load which happens to be F3's Zin)

The preamp mentioned in post #110 will work great with 6-12dB gain and it can put out 8V RMS into 10k load.
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Old 27th March 2012, 07:15 AM   #117
tp147 is offline tp147  Germany
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OK, thanks for reply juma.

I will check some params out, may be there is something wrong with the F3.
I only changed the power amp from NAP250 to F3. The NAP 250 has a gain arround 30 dB and with the NAC72s volume nob at 10-11 o'clock it is louder than with the F3 at full turned on

thanks & cheers

Thorsten
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Old 27th March 2012, 09:57 AM   #118
juma is offline juma  Serbia
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Quote:
Originally Posted by tp147 View Post
...The NAP 250 has a gain arround 30 dB and with the NAC72s volume nob at 10-11 o'clock it is louder than with the F3 at full turned on....
That's completely OK - NAP250 delivers a lot more power (80W at 8 Ohms) than F3 (15W). If your loudspeakers are not very efficient (more than 95dB/1W/1m), the F3's low power is the main cause of your dissatisfaction and no preamp will cure it. And if your speakers happen to have 4 Ohms impedance the situation will be even worse - at 4 Ohms the NAP250 will deliver more power and the F3 less (10W at 4 Ohms).

Last edited by juma; 27th March 2012 at 10:12 AM.
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Old 27th March 2012, 10:48 AM   #119
tp147 is offline tp147  Germany
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I guess the main problem are my speakers arround 90 dB!
I'll build some speakers > 95 dB soon, driven fully active, so I gather some poweramps right now and a preamp with active x-over.

I'm not really unsatisfied with the F3, but it was a big surprise when I connected first time and I raised my eyebrows due to the low volume - I didn't expect this in this range.

I'm not used to hear music at the highest levels, but this would be a little too less

thx & cheers

Thorsten
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Old 29th March 2012, 10:15 AM   #120
evanc is offline evanc  United States
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Juma,
I have been looking at yor circuit post53 and I think I have a spot where it will help me. I now have a dcb1 hooked to my F5 and for all the sources except the iPad I have plenty of gain. My idea is to build your circuit and put it between one input and the b1. The power can come from the dcb1. I'll leave off the volume pot and the input cap. Now my question, and I'm sure this will show my inexperience. In the schematic the source of the 2sk170 is hooked throughout the potentiometer to the drain of the 2sj74. In the copper trace from post 3 it looks like the 2sj74 is reversed. Same for the output pair. Where am I going wrong in my understanding?
Evan
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