Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Topics - J M Fahey

This way it´s easier to understand:

Suppose a pickup loaded by a 1M potentiometer.

We use a true 1M Log pot and a 1M Lin one with 100k strapped from wiper to ground.
We calculate 3 positions to make it easy and in any case are more than enough to show the heavy loading effect and wildly varying load impedance shown by the fake/simulated one.
A terrible idea I might add, sadly wildly popular because it looks "easy" .
I´ll simplify some values to closest round number, the idea here is not to get numbers accurate to 6 decimal places but to understand the concept of what´s happening.

Ok, let´s start with wiper on 0

True Log shows pickup load=1M , signal at wiper 0
Fake Log shows pickup load=1M , signal at wiper 0

Hey!!! it looks good!!!! ;)

Repeat with wiper on 5

True Log shows pickup load=1M , signal at wiper 11% 
Fake Log shows pickup load=590k , signal at wiper 18%

Mmmmhhhh, load has varied significantly,  maybe it won´t hurt pickup sound that much, now if pot were after a small coupling cap (as in VOX and Marshall) I would be losing almost the full lowest octave. :o
And Log simulation is not that good either, we are almost 6dB above what a true Log would offer.

Repeat with wiper on 10

True Log shows pickup load=1M , signal at wiper 100% 
Fake Log shows pickup load=90k , signal at wiper 100% .... IF pickup or earlier stage can happily drive 100k , 10X smaller than expected.

If it were a volume control after a small coupling cap, a common trick in respected Guitar amps, equalization would be a mess.

Same if following a tone control stack.

If following a triode stage, you would lose at least 6dB gain, because plate load resistor (typically 100k to 220k) would now be in parallel with 90k.

If following a pentode stage, which has high internal impedance and typically drives a high value load (220k and 470k are common values) effect will be a mess, easily losing 15dB signal.

Not enough time now to show the effect of a fake Log Pot used as variable NFB gain control in an Op Amp circuit, but believe me it´s even worse than this.

But is it absolutely useless then?
No, there is *one*  very limited case where it works, sort of, at least as a Saturday afternoon stopgap until shops open on Monday and you can buy a proper Log one: IF you use it as a passive Volume control after a low impedance driving stage, say an Op Amp, which can easily drive the much reduced impedance shown on 10 , then it´s acceptable.
Not accurate but human ear is so imprecise that it won´t complain.

Not even those who "can hear the influence of power cable Oxygen content" or whatever :D
Amplifier Discussion / Light Bulb Limiter
March 17, 2011, 12:23:04 AM
Some images:

I don't like the switch bypassing the bulb shown in the lower image, simply because it's too easy to switch it on by mistake.
Suggested bulb: 25W for small SS amps (10/30W) ; 100W for high power ones (60/200W); a tube amp might require a 150W to 200W bulb, simply because it eats too much on idle, because of all those filaments.
Most sites state that the bulb burns if a short is present; it does not; worst case it shines brightly, with full power.
If the amp is fine, the bulb usually lights barely visible red.
It allows to measure many things (do I have power rails? do I have DC on the speaker out?) which a fuse would not let you to, because it burns in a millisecond.
Remember all voltages will be low, the bulb is eating part of the wall voltage, do not aim for "schematic perfect" voltages but a reasonable proportion.
Example: power amp /main PSU states +/-40V DC
If I have a dark red or weak orange bulb, measure +/-25V, have no DC on output jack, amp is probably fine.
If I measure +/- 1 or 2V rails or +/-25V but have 24V on speaker out: bad sign.
In this last case, if I have 49V across the lower output transistor and 0.12V across the upper one, I can be pretty certain that the upper one is shorted, which will be confirmed later with an ohmmeter test with the amp off .
That's the general idea.
Much faster and better than:
replace one transistor -> power on -> blown fuse ... replace another -> power on -> blown fuse ... (repeat 25 times) power on -> nothing burns ... so THIS was the bad transistor !!!!  :grr :grr :grr
Amplifier Discussion / Fahey Amplifiers
January 07, 2011, 12:21:28 PM
Note: split from another thread

Hi joecool, thanks.
That's not surprising.
For long times I had a small factory running, with 3 to 6 guys working for me, with Christmas and New Year peaks of up to 8.
Now I'm working with just one guy who does the carpentry, transformers, mounts speakers, etc.
I make boards, panels, speakers, plus actual wiring.
My amps are very simple, I can wire 4 100W heads a day.
In Argentina it was *impossible* to buy US made and British amps, so people were happy to buy whatever I could provide, as long as I had the same quality sound.
When economy "opened" we were flooded with cheap imports and a big part of Argentine economy died, specially the Industry.
I survived because I strayed away from "expensive/boutique" amps (nice but I might sell 8 a year) and got into good sounding but very competitively priced "battle proven workhorses".
I do not make "student" amps, I can't compete with chinese cheapies, 95% are 100W "stage/live" amps.
As an example: my 100W head (Guitar or Bass) costs U$140 ; my 2x12" cabinet the same

New, with 1 year guarantee, parts and service assured for the next 10 or 20 years (that's a big selling point).
That's why I sell around 20 a month, with lows of  8 and highs of 36.
I've had many 6 amp Saturdays , with clients arriving with 1 to 2 hour intervals so they can test freely.
There have been bands born, with musicians meeting here, jamming together and liking what came out.
Just for some old photos and scans:
It's in Portuguese, this was hastily thrown together for some Brazilian customers.
The magazines are the local equivalent of Guitar Player.