Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum

Message Sort: Post Order or Asylum Reverse Threaded

Removing the input coupling caps??

207.62.246.184
Posted on August 19, 2014 at 13:22:00
Tweaker456
 The designer of this amp and another electronics person says there is no need for the input coupling cap because of the extreame unlikelyhood of dc on the input. Someone is telling me not to remove it because the amp could go unstable. So what's up? Did the designer put these basterds in because of an abundance of caution, it really being unnecessary for modern inputs?? I'm not an electronics person, just a country tweaker. Thanks, Tweaker

 

Hide full thread outline!
    ...
RE: Removing the input coupling caps??, posted on August 19, 2014 at 13:39:08
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 If you want an intelligent critique of someone else's design, you should at least include some information on the design to be critiqued. Maybe even a circuit diagram?

 

RE: Removing the input coupling caps??, posted on August 20, 2014 at 05:06:39
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012

An input cap blocks DC and it also forms a high pass filter. Many designs from the 50s and 60s had input caps. In the above schematic, the 0.1uf cap and the 33pf are filters. Could you remove them, probably as CD players do not have wide-band noise problems. Should you? You can do whatever you want, modern designs typically do not have input caps unless the first stage is a cathode follower. It is unlikely to blow anything up.

 

RE: Removing the input coupling caps??, posted on August 20, 2014 at 08:16:28
Hornlover
Manufacturer

Posts: 2529
Joined: March 8, 2002
 If the previous stage has an output coupling cap (probably does), then you can remove the one in the amp. You dont need two.

 

Input Cap Functions, posted on August 20, 2014 at 08:37:43
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 The 0.1 is not intended to be a filter, although it does create a slight rolloff in the 10Hz region. Its actual purpose is to block any DC that's present at the source. The 33pF/470k combination is a low pass filter to attenuate RF (broadcast stations, ham radio operators, etc.) It's a little too close to the audio band for my taste, and I would remove it. A 200kHz LC low pass would be a better choice, but only if needed in order to solve a problem in a particular situation.



 

RE: Input Cap Functions, posted on August 20, 2014 at 12:29:39
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 I was addressing any possible stability effects in addition to the DC blocking.

 

I wasn't looking for a critique..., posted on August 20, 2014 at 13:37:22
Tweaker456
 It's tube headphone amp with one 5670 per channel going out to the transformer. Tweaker

 

High pass filter aspect..., posted on August 20, 2014 at 13:40:39
Tweaker456
 This removal could in theory allow for hiss type noise in. Thanks for the heads up on this. I'm a bit rusty. Can you tell me what is the crossover point on a .1?? Thanks, Tweaker

 

Rolloff freq., posted on August 20, 2014 at 13:43:18
Tweaker456
 Looks like triode kingdom answered the ? 10k Tweaker

 

But that's what you asked for..., posted on August 20, 2014 at 17:00:27
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 "Did the designer put these basterds in because of an abundance of caution, it really being unnecessary for modern inputs??"

The answer to that requires a technical critique of the circuit. And, as Paul said, no one can do that unless you take the time to provide a schematic (and maybe answer a few questions after we look at it).

 

You can't talk about the 0.1uF cap in isolation,..., posted on August 20, 2014 at 17:45:27
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 it's part of a network. Simplistically, the 0.1uF cap in conjunction with the 470K resistor would form a high pass filter with a -3db point around 4-5 Hz (too lazy to calculate). However, the 47k resistor in series, in conjunction with the little pF value cap in parallel together form a low pass filter, apparently for RF, per TK's post.

 

RE: Removing the input coupling caps??, posted on August 20, 2014 at 22:05:34
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 Doesn't the cap in series and the resistor to ground form a line crossover point so the amp only sees an input frequency of whatever the two form?

 

SPICE, posted on August 21, 2014 at 08:29:39
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Here's the SPICE plot. The solid line is amplitude; dotted line is phase. It indicates a 1dB loss at 1kHz due to the 5K internal resistance I added into the generator. That did not otherwise materially affect the response.

Note that the high end rolloff and phase shift will be greater than shown due to the input capacitance of the tube.




 

RE: I wasn't looking for a critique..., posted on August 21, 2014 at 12:17:07
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 That's still not enough information.

For example, maybe the cathode of the 5670 is grounded, and a battery is used to bias the grids. If this is the case, I would leave the cap in.

If the cathode is biased with a resistor, LED, transistor, or regulator, then there's certainly the possibility that you can remove the capacitor without any harm.

 

RE: Removing the input coupling caps??, posted on August 24, 2014 at 15:22:31
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hey tweak, I always remove them from my rebuilt amps but there is 1 situation where if I had some installed in one of my amps it would have saved it from a catastrophic event. The amp was a Hybrid ss with tube front end powerful 200 w/channel. My son used it as a music amp for jamming. When he plugged in his ancillary equipment which was outputting significant DCV the overvoltage just fried all the output transistors on the amp. If there was a blocking cap on the input that would not have happened. Just be careful who you lend your amps to. cheers, Dak

 

My correction to this (my) comment..., posted on August 25, 2014 at 13:05:09
Tweaker456
 no hiss , I mixed up hi and low pass.

 

Page processed in 0.029 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]