Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on June 26, 2012, 09:12:41 am
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My ampeg preamp sounds awesome with the ultra low engaged but not so much so disengaged -
the amp sounds pretty knarly and grainy thin instead of having any balls left at all.
I am pretty sure its an easy fix but whats bugging me is I cant get my head around the schem.
I am stuck understanding the finction of the switch effecting the signal bath between R7/C3 and P1.
My novice eyes tell me the ultra low switches between R11/C4/R12 and maybe R9/C5/R10/C6
But the more I look at it it looks like that stuff is always in the in the circuit, the switch just gives the signal a direct route to the volume pot P1 from R7.
So I am wondering if there is something to do with the signal taking the path of least resistance that makes it go straight to the vol pot or not?
The fix isn't so much of an issue I am sure if i look at those components i'll dig up cure but I don't like that I cant see or understand the path of the signal in the schematic!
any schenmatic 101 help gratefuly appreciated!!!
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My ampeg preamp sounds awesome with the ultra low engaged but not so much so disengaged -
the amp sounds pretty knarly and grainy thin instead of having any balls left at all.
...
I am stuck understanding the finction of the switch effecting the signal bath between R7/C3 and P1.
Switch 1A is but one pole of Switch 1. The SVT 2 Pro schematic in Hoffman's library didn't match your fragment, so it would be helpful to see what else is being switched. SW1A is also a double-throw switch. Let's look further...
The lower of the three circles is the switch contact, and it alternates between the upper two circles depending on switch setting. As drawn in your schematic, C3 is being connected to P1, so the Ultra Low circuit is being bypassed. This setting probably corresponds to the sound you don't like.
If the switch is thrown the other way, the Ultra Low circuit is brought into play. This circuit is a Twin-T Notch Filter (http://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php), and is made up of R11, R12, C4 (T #1), and C5, C6, R10 (T #2).
In theory, a Twin-T filter can achieve infinite rejection of a single frequency. Reality is that unless the resistors and caps are very carefully matched (and usually require trimming caps, etc), the cut at the center frequency won't be infinite and the sides of the notch have a somewhat shallower angle. If you're used to parametric eq terms, the real circuit doesn't have as high a Q as the theoretical circuit.
Ampeg went a step further; to exactly match the Twin-T topology, C4 should be twice the value of C5 and C6. Ampeg used the same value for all three, which widens the notch a bit. Plugging the values into a calculator for this circuit, the center of the notch is around 590Hz and has some slight roll-off all the way down to 10Hz. 600Hz would be basically your treble when playing bass, so activating the Ultra Low circuit is really chopping out the treble.
However, we still don't know what other element is being switched when SW1A is thrown. Look over the schematic for any other "SW1_" designators (like SW1B). That will tell you what else is being switched when you manipulate the Ultra Low switch.
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Thanks HBP
I should have said that S1A and S1B while the same physical component they have seperate lugs and seperate 'elements are switched' with the two switches attached - one engages the ultra low and one switches from pre to post eq for the DI out of the amp.
What I was trying to figure out is what components are NOT being used when the ultra low circuit is engaged. as there is definately something wrong when just using the standard low circuit.
It is not only that i don't like the sound of the standard low but it is more than that there is something wrong.
thanks again
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What about R37, C24? How can they do anything if their grounded at C2?
There're between ground and ground. :dontknow:
Brad :think1:
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If the switch is thrown the other way, the Ultra Low circuit is brought into play. This circuit is a Twin-T Notch Filter (http://www.radio-electronics.com/info/circuits/rc_notch_filter/twin_t_notch_filter.php), and is made up of R11, R12, C4 (T #1), and C5, C6, R10 (T #2).
Ah ok - So I must be looking in the wrong place as i thought they were two seperate sections switchen in or out but as you say the whole lot of that section is either in or bypassed straight to the vol pot.
So back to the start.
Problem = weak and grainy bass signal mids and highs fine - problem is rectified by engaging the ultra low circuit but obviously this adds in alot of extra low i dont want.
The fact that it sounds so good with the ultra low engaged leads me to beleive the problem could be between the input and the volume pot -
any ideas?
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The fact that it sounds so good with the ultra low engaged leads me to beleive the problem could be between the input and the volume pot
Or, Ampeg designed it to sound good with the Ultra Low engaged. So it sounds like sh!t without the Ultra Low.
Kinda like a reverse of Fender's Deep switch, which destroyed highs when you activate it (come to think of it, about the same).
You'll either like iot one way or the other. Or, maybe you'll like the Ultra Low with a bass, but not-Ultra-Low with a guitar.
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hmm you could be right I guess.
I can/do use it a way as it is and its great.
But the sub-bass gets in the way more than i'd ideally like - its like everything has a dub overtone to it! and if i don't use the ultralow it sounds a bit farty!
I have gone through it and nothing jumps out as being wrong wrong so maybe i'll give up on it and put it down to design.
The only other weird thing is that the problem doesn't seem quite as pronounced on the bright channel but then again the normal channel is much much louder than the bright.
ugh - I don't know!
thanks