Got my clone fired up yesterday. This one is dual mono in a 3U Modushop chassis with old BrianGT power supply boards. The 15 Volt toroids were initially intended to be used for a mini-aleph build, so the PS rails are only +/- 19V. My (very) initial impression is very positive with only basic adjustments to set bias and offset. Many thanks to everyone here, especially @Nelson Pass, @lhquam and @mareli.
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Cool and lovely build, congratulations. Could you measure the output impedance of your amp? There is a help for easy calculations here: http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm
I did 2 measurements at 1kHz, one with an 8 Ohm load and the other with a 2 Ohm load. Measured this way, Zout calculates to 0.088 Ohms. Give or take a bit for resistor tolerances.
Thanx for the measuring. It seems very, very close to the specified Zout = 0.08 Ohm, so you have chosen right bias for 19V rails.
My clone has Zout = 0.42 Ohm measured with 8 and 4 Ohms at 24V rails and 1A2 bias. The method is the same and it doesn't depend on
what load resistors are used.
I believe the transconductance of the Exicons I used plays some role here. May be I am wrong?🤔
My clone has Zout = 0.42 Ohm measured with 8 and 4 Ohms at 24V rails and 1A2 bias. The method is the same and it doesn't depend on
what load resistors are used.
I believe the transconductance of the Exicons I used plays some role here. May be I am wrong?🤔
I dunno. FWIW, I'm currently running this off my variac cranked all the way up, so my rails are at +/-22V, and biased at 1.4A. I'll be swapping out those 15V donuts for 18V ones sometime soon.
Hi all,
I'm in the process of converting my F5 type amp to an F7 type and I am curious about one thing:
When the First Watt F7 damping factor is rated @100 do we know the preamp output impedance that is used to arrive at this figure ?
thanks
mike
I'm in the process of converting my F5 type amp to an F7 type and I am curious about one thing:
When the First Watt F7 damping factor is rated @100 do we know the preamp output impedance that is used to arrive at this figure ?
thanks
mike
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The preamp has nothing to do with it, its the ratio of the output impedance (of the power amplifier) to the load impedance (of the speaker). You can work this out quite easily in the simulation of the amplifier.
To be totally honest in this specific circuit the preamp has some influence given the positive feedback to the input node, but it's usually swamped by the much higher gate resistor. It might become important if you have a tube preamplifier with 10s of kohm output impedance (yes, some manufacturers here still have these kind of offerings available for sale for some obscure reason).
Hi Sangram,
Sorry, but this is simply not the case. I have been experimenting extensively in spice and the F7 type amp's DF is entirely dependant upon the O/P impedance of the Pre-amp.
Also, I watched a you tube review in which it is reported that Nelson said that the F7 is really designed for valve type Pre-amps. This was revealed after an attempt to use a line o/p from a Hegel power amp which resulted in the F7 sounding bad.
Here's an example from spice where the only variable was the Preamp O/P impedance:
Pre-amp O/P impedance = 2 ohms the F7 type amps damping factor = 9
Pre-amp O/P impedance = 725 ohms the F7 type amp's damping factor = 191
If the F7 input is strongly tied to ground the Positive FB feed cannot change in the F7 input signal enough to make any difference to the DF
So if anyone knows the pre-amp O/P impedance used to generate the official spec of DF = 100 I would be interested to hear what it is.
thanks
I'll relook at the sims, thanks for your findings. I don't remember a massive issue either way, but then I was using a transformer-coupled input. Logically what you say makes sense, but I do have to terminate the transformer with something close to an expected load in my case.
If this is the case, the input resistor in the amp can be increased by that amount, for a low impedance source.
That's a better location for the resistor, anyway. However, a tube preamp will typically have a higher Rout than 725R.
You don't want to bring the sensitive input node, where the feedback is sensed, out of the amplifier case.
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If a tube pre-amp has a higher O/P R out then we can expect that the F7 damping factor could very easily be negative.
This means that when the speaker is connected to the amp, the amp O/P actually increases.
The F7 really needs an input buffer (before the series feedback resistor) to be compatible with most preamps.
If you look at an actual F7, you will find that the series input resistor is 39K, which is very much
higher than any source impedance you are likely to encounter, plus there is a limiting shunt
resistor before that which places an upper limit of 10K for apparent source impedance even for
open circuit.
If someone doesn't like the sound, that's OK by me as they seem to be in the minority.
higher than any source impedance you are likely to encounter, plus there is a limiting shunt
resistor before that which places an upper limit of 10K for apparent source impedance even for
open circuit.
If someone doesn't like the sound, that's OK by me as they seem to be in the minority.
Many thanks Nelson - that is food for thought.If you look at an actual F7, you will find that the series input resistor is 39K, which is very much
higher than any source impedance you are likely to encounter, plus there is a limiting shunt
resistor before that which places an upper limit of 10K for apparent source impedance even for
open circuit.
If someone doesn't like the sound, that's OK by me as they seem to be in the minority.
Should make it relatively insensitive to whichever pre-amp is used
Strange then that Jay said it did not sound that great with Hegel as a Pre ( Perhaps it's was just the Hegel )
He said that everything else sounded great.
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My F7 simulation says that if the source impedance Rgen decreases, then the DF increases.
However, the DF does not change too much😉
