Hello again,
Seeing as how my first question/thread garnered adequate attention and qualified help, I'll try another. I did a light search for threads throughout this forum and did not find anything of much help to my current situation.
For my clavinet project [see thread: http://www.diyaudio.com/forums/inst...ickup-wiring-inductance-output-questions.html] to go further, I needed an amp to properly test the circuit. I ended up finding a used solid-state guitar amp that had enough control [and high enough input impedance] to use with the clav's circuit when it's finished - for the time being, I've got a cheap guitar to plug into it and noodle with until the clavinet is ready.
It is a Rocktron R50C - solid-state 50W, 2x8", stereo chorus, stereo ext-outs [8 Ohm Min.]. Pretty run-of-the-mill, until the stereo chorus is switched on, which [obviously] provides the sound with pleasant depth and breadth - anything from basic stereo audio effect to a warbling vibrato. Overall very neat, and worth more than what I put down for it. Essentially brand-new, and most of it works without error - very much a keeper IMO. In fact, with the stereo extension-speaker outputs, I may eventually construct separate 'stereo' speaker cabinets to take advantage of that capability [with adequate preliminary reading/forewarning, of course].
Anyways, here's my problem: the only components that don't operate as they should would be the entire EQ section - the basic rotary-pot 4-control "Low-Mid-High-Presence" array. The signal path is not interrupted in any way, but the specific band that its respective pot should control has been 'scrambled' or 'reassigned' to another, I guess. This is what is occurring [control name, followed by result]:
Is there a simple explanation for the aforementioned EQ behavior? Something that I can pinpoint to begin with, and see if it makes a difference? I would REALLY love to get this amp working completely. For the record, I have [since my last question] looked into basic precautions and safety-first guidelines [including filter cap drain] concerning the insides of an amplifier.
Thanks-in-advance for all of your comments, questions, criticisms, and assistance. Also, I can take detailed pictures of the circuit board inside and post/link them, if necessary.
Seeing as how my first question/thread garnered adequate attention and qualified help, I'll try another. I did a light search for threads throughout this forum and did not find anything of much help to my current situation.
For my clavinet project [see thread: http://www.diyaudio.com/forums/inst...ickup-wiring-inductance-output-questions.html] to go further, I needed an amp to properly test the circuit. I ended up finding a used solid-state guitar amp that had enough control [and high enough input impedance] to use with the clav's circuit when it's finished - for the time being, I've got a cheap guitar to plug into it and noodle with until the clavinet is ready.
It is a Rocktron R50C - solid-state 50W, 2x8", stereo chorus, stereo ext-outs [8 Ohm Min.]. Pretty run-of-the-mill, until the stereo chorus is switched on, which [obviously] provides the sound with pleasant depth and breadth - anything from basic stereo audio effect to a warbling vibrato. Overall very neat, and worth more than what I put down for it. Essentially brand-new, and most of it works without error - very much a keeper IMO. In fact, with the stereo extension-speaker outputs, I may eventually construct separate 'stereo' speaker cabinets to take advantage of that capability [with adequate preliminary reading/forewarning, of course].
Anyways, here's my problem: the only components that don't operate as they should would be the entire EQ section - the basic rotary-pot 4-control "Low-Mid-High-Presence" array. The signal path is not interrupted in any way, but the specific band that its respective pot should control has been 'scrambled' or 'reassigned' to another, I guess. This is what is occurring [control name, followed by result]:
- Low: nothing happens. From '0' to '10', no perceptible difference in signal.
- Mid: controls the lows. No midrange difference.
- High: very vague, almost imperceptible difference in super-high-frequency range [as if it were the 'Presence' control]. Normal treble range unaffected.
- Presence [super-highs]: dramatic reduction of treble overall, no midrange difference, with bass retained - entire signal muffled on zero, as if it were a 'Tone' control on a guitar circuit.
Is there a simple explanation for the aforementioned EQ behavior? Something that I can pinpoint to begin with, and see if it makes a difference? I would REALLY love to get this amp working completely. For the record, I have [since my last question] looked into basic precautions and safety-first guidelines [including filter cap drain] concerning the insides of an amplifier.
Thanks-in-advance for all of your comments, questions, criticisms, and assistance. Also, I can take detailed pictures of the circuit board inside and post/link them, if necessary.
I agree with Bone, an electric guitar has usually a relatively loud mid-range, 400Hz may be 9 to 12dB louder than 100Hz. The amps response is tailored to make this flat again (with the EQ centered).
There may also be a low pass filter in place (10-14kHz), that you can not influence with EQ.
In addition the amps speaker may go only to about 12-16kHz.
Using aux input could improve things, but it might go straight to poweramp, bypassing the effects section.
There may also be a low pass filter in place (10-14kHz), that you can not influence with EQ.
In addition the amps speaker may go only to about 12-16kHz.
Using aux input could improve things, but it might go straight to poweramp, bypassing the effects section.
And on a different note, weren't the original guitar amplifiers used to reproduce voice along with accordion and guitar? They could not be all that out of whack. Also what real frequency content is there above 14 kHz? And pushing 6 kHz is a lot for a guitar speaker. And as far as a guitar having the 400 Hz region being 10dB louder than the fundamental of the open strings, well where does all that flubby overdrive sound come from when using a output transformer that does not have adequate low frequency bandwidth? And even if the bandwidth was limited on a guitar amp why would the tone controls have no effect?
Not having a schematic to look at but seeing that the amp has a passive TMB tone control I would say you have a open connection at the base of the tone stack. That is how we effectively take the tone stack out of the signal chain, by lifting the ground on the mid pot.
Not having a schematic to look at but seeing that the amp has a passive TMB tone control I would say you have a open connection at the base of the tone stack. That is how we effectively take the tone stack out of the signal chain, by lifting the ground on the mid pot.
You might be surprised. Check out this neat little app:
TSC
To get "flat" response from most guitar amps, you have to turn the bass and treble down all the way.
I use it all the time. The thing I was disputing was that
And the Marshall only has to be turned down to '4' to get a mainly flat response. The James is flat at '5' (as used by Traynor and others), The 'E' stack which is mainly flat with bass at '5' and treble at '6') although this tone stack is not used much other than in some vintage gear and some pedals), And the Big Muff type of control is normally used in pedals. And there are those who tweak it increasing the high pass section capacitor to 9nF removes the mid dip with a response more suited to the 18 Watt (with values scaled for tube circuits). And then there are the Tweed type of amps that are not represented here.
Which does not change the OP's question, 'Why does my tone controls have no effect on the sound?' Even if the frequency response is not flat (and who says that is desirable?) in an amp, the tone controls should effect the sound.
Presence in tone controls is usually the upper-mid range (or "low treble")
FWIW, My old Marshall JCM800 and Fender Princeton guitar amps are "far from flat" when all tone pots are at 12 o'clock.....
YMMV
Presence controls are usually the result of reducing the feedback of the power amp by rolling off the treble in the feedback signal. Because there is less voltage to reduce the treble signal coming into the power section there is more treble at the output. Has more effect when running the power amp clean than when in clipping.
Thanks for the technical explanation. Sonically, my impression is that Presence affects the "lower treble" range....
YMMV
You are perfectly correct, presence is just a mid-range control.
I suspect Printer2 has seen a design that implements this in a feedback network?, although most guitar amps seem to use passive ones.
No, in a guitar amp 'presence' is a treble control. Printer2 has seen Fender and Marshall schematics. They pretty well define "most" guitar amps.
(BTW, if you're going to use Google/Wikipedia to provide answers, please click on the link and actually read the Wiki article, don't simply depend on the first line Google displays.)
(BTW, if you're going to use Google/Wikipedia to provide answers, please click on the link and actually read the Wiki article, don't simply depend on the first line Google displays.)
Peavey does it also and also has a resonance control in the feedback loop. If you look at schematics with this control you will see a capacitor shorting out the feedback loop to ground. There is no possible way it can effect the 'lower treble' and not effect the 'upper treble'.
Pls. see this circuit diagram of a Marshall amp (Super Lead series, type 5010, solid state amp from early 80s). IMHO Presence here is just a kind of mid/treble control and not in the power amp's feedback. With C15 to GND there's some treble cut, and with C15 parallel to R11 there's some highpass filter. Pls. correct me if I'm wrong
Perhaps tube amps have different Presence circuitry?
YMMV
Then you obviously already know that those articles state it's an upper-mid control - with the original Fender ones doing so in the negative feedback.
See any there mention of mids being controlled? I don't.Wikipedia said:
They do, although some later Marshalls have something similar in their tube preamps.
However, the point should not be "let's find some exceptions to the rule" but rather, "let's understand where this term came from and what it's generally accepted meaning is".
Traditionally, a "presence" control on a guitar amp affected the highs, mostly above the range of the treble control. This is different from the way the term is used in hifi and recording, but it's fairly common for musicians to have their own meaning for terms. Consider "gain", for instance.
)Pls. see this circuit diagram of a Marshall amp (Super Lead series, type 5010, solid state amp from early 80s). IMHO Presence here is just a kind of mid/treble control and not in the power amp's feedback. With C15 to GND there's some treble cut, and with C15 parallel to R11 there's some highpass filter. Pls. correct me if I'm wrong
Perhaps tube amps have different Presence circuitry?
YMMV
Oh, a SS amp. That is so sad.
I guess they had to give a function to the presence knob in order to justify it.
Fender 1958 Bassman, 1959 Marshall has the same circuit with a few value changes.
Peavey
An externally hosted image should be here but it was not working when we last tested it.
Other tube amps with a presence control basically do it the same way.
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