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Limiting low frequency response of SE amp

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Posted on February 4, 2017 at 06:13:10
Randal P
Audiophile

Posts: 33
Location: NYC Area
Joined: August 19, 2004
 I have a single ended amp (Dennis Had Inspire, single ended 6V6). I wish to use this to power the high frequency section of my Altec two way (414A, 802D). I would like to roll the amp off at 6dB/octave below 6khz or 8khz or so at 6dB/octave.

One option is the simply insert a capacitor inline with the input, but I think I would need to know the input impedance to calculate the value. Another option might be to select a smaller coupling capacitor value to limit LF response.

Any suggestions?

 

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RE: Limiting low frequency response of SE amp, posted on February 4, 2017 at 06:17:40
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Rolling off at the I/P is very easy. "Plug" the grid to ground resistor value into the RC filter formula.


Eli D.

 

RE: Limiting low frequency response of SE amp, posted on February 4, 2017 at 06:56:41
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 assuming the amp has at least 1 coupling cap in it then that is the best place to do it. Simply adding another cap for a filter when chances are there are several of them already in place is not a very well thought out plan of action.

Along those lines the choice of coupling cap and resistor combo is a general balancing act with a choice of compromise related to the lowest frequency of interest. Taking a needed low frequency response of say 2hz out of the picture and moving it up a few decades opens up a number of possibilities for sonic improvements.

If you post the schematic or list the tube compliment and coupling cap / following resistor to ground sizes i'm sure a good solution will become obvious.

dave
dave

 

RE: Limiting low frequency response of SE amp, posted on February 4, 2017 at 08:27:43
Randal P
Audiophile

Posts: 33
Location: NYC Area
Joined: August 19, 2004
 It is the same as this guy: http://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=259009

Dennis doesn't provide (or even use) schematics and my tube amp building knowledge is rudimentary at best.

 

RE: Limiting low frequency response of SE amp, posted on February 4, 2017 at 12:15:34
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



RC filters follow a fairly simple formula.

(2Π) (Cfarads) (Rohms) (Fhertz) = 1

Any 2 of the 3 variables define the 3rd.

In the case of "El Cheapo", the 0.056 μF. cap. and 100 Kohm resistor "corner" at 28.4 Hz. Here, the filter is high pass.


Eli D.

 

RE: Limiting low frequency response of SE amp, posted on February 4, 2017 at 18:23:45
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 I assume this is a cap-coupled design, but checking because I believe Dennis is a big fan of direct coupling.

If you modify the interstage coupling cap, the driver will still have to handle the full-range signal. It will overload long before the output tube overloads. Maybe you don't need much power, but the main (technical) point of multi-amping is to have more headroom. FWIW, at 60-80Hz the filtered signal is down 40dB, which is 0.01% of the fullrange power.

For that reason, placing the filter before the driver is much preferred.

In most cases, you can simply measure the resistance at the input RCA jack with a meter, and use that to determine the capacitance needed between the jack and whatever it connects to.

Mr. Had is often on this forum, hopefully he will weigh in on the question. Always best to get advice from the source :^)

 

RE: Limiting low frequency response of SE amp, posted on February 5, 2017 at 11:19:01
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 Hey paul,

assuming the driver is not inductively loaded I don't see how keeping LF info out of the picture changes anything wrt overload characteristics.

I don't see a tube caring about the low frequency content of the signal and as far as headroom goes you still need enough for the high passed signal to operate so by what mechanism does restricting bandwidth help?

dave

 

I assume that this is to feed a tweeter?, posted on February 7, 2017 at 18:20:34
Timbo in Oz
Audiophile

Posts: 23221
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
 If so, does the tweeter currently have a series capacitor to high-pass it in between 6-8 Khz?

If yes, you must remove that capacitor.

A line level input filter (change the input Cap) or changing the value of a coupling capacitor inside the circuit are both good ways to do it. The first is easy to calculate and even with a commercial metallised film cap will sound better than any speaker level cap filter ever will. ? Because it won't be working hard.

It might also have the advantage of easing the task of the input-gain stage.


Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger


 

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