Rig-Talk

Guitars, Gear, Tone!
It is currently Tue, May 22, 2018 1:18am

All times are UTC - 5 hours




Post new topic Reply to topic  [ 4 posts ] 
Author Message
 Post subject: Depthmod explained
PostPosted: Fri, Mar 16, 2018 5:48pm 
Offline
Plank Cranker Trainee

Joined: Sun, Sep 27, 2009 2:56am
Posts: 480
Location: Trapped Under Ice
Are you human?: 951
Anybody, hold nothing back when explaining this in technical terms please.

_________________
71' 100w Langner Superlead...Cameron tweaked, Cameron approved
Langner DCP-1 Elite preamp w/Hyper mod
5150 II Cameron modded
83' JCM 800 2203
79' 1960B Marshall Cab w/G12M-25 creambacks
Numerous Custom Jackson King V's

Sell me your Jackson KV1T :rawk:


Top
 Profile  
Reply with quote  
 Post subject: Re: Depthmod explained
PostPosted: Fri, Mar 16, 2018 6:41pm 
Offline
Plank Cranker Wanker
User avatar

Joined: Fri, Jun 10, 2016 9:08am
Posts: 1582
Location: Near beer
Are you human?: 951
Many amps take signal from the speaker jack and feed it back into the amp at opposite phase to sort of tame/tighten the power section and even out the frequency range. Since it is opposite phase it is negative feedback. If you flipped the phase it would be positive feedback the amp would squeal/roar like crazy as soon as you flipped the standby (this is not fun). The typical Presence control shunts some of the high end frequencies of that negative feedback to ground (so it isn't fedback). So as you turn the Presence up, the power section has less negative feedback at high frequencies and thus you get more high end. The Depth mod/Resonance control does a similar thing but for low frequencies. As you turn up the Resonance/Depth control, the power section has less negative feedback at low frequencies and the amp produces more low end.


Top
 Profile  
Reply with quote  
 Post subject: Re: Depthmod explained
PostPosted: Sat, Mar 17, 2018 1:55pm 
Offline
Immortal Rock God

Joined: Mon, Apr 07, 2008 9:49pm
Posts: 10480
Location: MD
Are you human?: 951
SpiderWars wrote:
Many amps take signal from the speaker jack and feed it back into the amp at opposite phase to sort of tame/tighten the power section and even out the frequency range. Since it is opposite phase it is negative feedback. If you flipped the phase it would be positive feedback the amp would squeal/roar like crazy as soon as you flipped the standby (this is not fun). The typical Presence control shunts some of the high end frequencies of that negative feedback to ground (so it isn't fedback). So as you turn the Presence up, the power section has less negative feedback at high frequencies and thus you get more high end. The Depth mod/Resonance control does a similar thing but for low frequencies. As you turn up the Resonance/Depth control, the power section has less negative feedback at low frequencies and the amp produces more low end.


This isn't quite correct.

I'm going to start with an explanation of what a phase inverter does and work my way forward. OP want's the truth, so this is the best I can provide:

A phase inverter serves the purpose of splitting the phase of the original AC signal and sending each phase to a separate set of power tubes to be amplified out of phase with one another. How much out of phase depends on the grid bias supplied to them, hence the term crossover distortion where the cosinusoids are not re-combined correctly. The primary of the OT is phased with the original input signal source in terms of winding direction, as the direction you wind a transformer determines the phasing (hence the dots on schematics). The outputs of the power tubes are then sent to the OT primary taps, where the final AC signal is transduced from a high impedance, high voltage, (read large amplitude), small current signal, into a low impedance, low voltage, high current signal needed to drive a low impedance load like another transducer (IE a speaker load). This is why the OT is needed for maximum power transfer of a matched secondary to the load's impedance.

Global feedback has two key parameters - a damping factor and a gain-bandwidth trade-off. More negative feedback reduces gain for a trade in increased gain-linearity over a specified bandwidth. Too much damping factor and the amplifier will not sustain a note or have much gain at all. Too little feedback and gain becomes non-linear over the entire bandwidth. For audio bandwidths such as with guitars, higher notes can end up being much louder than lower notes, or vice-versa depending on the bode-plot response and transfer function characteristics of everything included within the closed loop transfer function of the negative feedback loop.

This introduces another topic - feedback has to be applied carefully and designed accordingly. The transfer function of the circuit has what are called poles and zeros - poles in the denominator of the closed loop gain transfer function and zeros are in the numerator. If you have a damping factor that is less than your bandwidth, you're in the dangerous territory of introducing ringing, oscillations, or frequency-dependent instabilities like motorboating. This is caused by having your poles of the circuit within the RHP (right half plane) of the s-plane plot (damping factor = x-axis and frequency = y-axis).

NFB is applied by taking a tap on the secondary of the OT which is out of phase with the primary and wiring it into the out-of-phase-input of the phase inverter. At that input stage, there is a series-shunt resistor combination. The ratio of two resistors helps to determine the overall amount of negative feedback and characteristic response of the closed loop gain transfer function created by the global negative feedback. The shunt pair depends on the design of the phase inverter, and the series resistor determines how much NFB is used. The tap you use on the OT does not matter, as the series-shunt resistor combinations can be changed to achieve the same amount of negative feedback for a different tap from the OT. In reality, it's a combination of all three that determine the overall closed loop gain of the transfer function.

Now, with regards to the depth mod, you can put capacitors in parallel with the series resistors to create an RC network and introduce a pole in the transfer function. The size of the resistor and the capacitor will determine where the pole exists. For stability, you want the pole to exist outside of your bandwidth of the amplifier and in the LHP of the s-plane plot, else the circuit will go unstable at that frequency location.

It's fairly challenging to verify stability analytically, and I'm not going to get into this topic on RT as we would need some type of latex editor for clarity. However, most people just copy designs of previous years because they know it works. Very rarely will you see someone design a new PI stage from scratch using tubes these days, albeit can be done.

So there you go - depth mods and how it applies to the amplifier design, universus.

_________________
Amplifiers:
5150 II Head w/MM Iron
Henning Cherry Bomb #22/37

Guitars:
2012 Fender Select
1988 Charvel 475XL Special
2018 Gibson Les Paul Traditional

Effects:
ADA 2FX
Lexicon PCM 81
TC Electronic 2240
Damage Control Timeline
Dunlop 535Q


Top
 Profile  
Reply with quote  
 Post subject: Re: Depthmod explained
PostPosted: Sat, Mar 17, 2018 7:52pm 
Offline
Plank Cranker Wanker

Joined: Wed, Sep 02, 2015 12:47pm
Posts: 1485
Are you human?: 951
I’ll make it easy. It adds in bottom end. You can adjust the frequency by changing the cap you use on the typical 1 meg pot. .001uf is more Marshall sounding and .0047uf is more sub lows. I prefer .003uf. I’ll add that cap type tonally matters depending on the circuit. Personally, with Marshall type amps I would use a ceramic cap. There are other tips and tricks to do here, but this is the basic.

_________________
Please don’t PM me any questions about any amps since I only check messages when selling gear.


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group