Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Vented box for bass guitar design - HELP!
Vented box for bass guitar design - HELP!
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th October 2017, 11:11 PM   #21
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by chris661 View Post
Would it be better to say the output impedance effectively raises Qes?

Chris
To me, there are the three Thiele-Small parameters Qes, Qms, and Qts, and then there is the actual mechanical Q of the driver, mounted in its enclosure, and connected to an amplifier. The last one is not a Thiele-Small parameter, but simply the Q-factor of a damped mechanical harmonic oscillator, as outlined in a thousand basic physics textbooks.

I think we all agree that this actual mechanical Q is affected by several factors, including Qes and Qms, the speaker enclosure volume (if sealed) and air mass in the port (if vented).

There are two limiting values for this mechanical Q that I found helpful in understanding how speakers work. One is that, if the amp's output impedance is essentially zero, the unmounted drivers free-air Q is virtually equal to Qts.

The other is that, if the amp's output impedance is much greater than Re, the free-air Q is virtually equal to Qms.

-Gnobuddy
  Reply With Quote
Old 26th October 2017, 04:18 PM   #22
chris661 is online now chris661  United Kingdom
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
I understand what you're saying, but I think for the sake of clarity we should avoid calling it "mechanical Q", since Qms has that definition.

I quite like Qtc, ie, cabinet total Q.

With regards to output impedances, valve amps are usually a couple of ohms. I haven't heard of anything outside current drive where it's very much greater than Re, where, as you note, Qts (total Q of the speaker) would be Qms.

Chris
  Reply With Quote
Old 27th October 2017, 03:21 AM   #23
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Qms is the mechanical Q due only to the speakers own suspension and moving mass - spider & surround compliance, cone and voice coil mass.

The "Q" I was talking about is the actual mechanical Q of the speaker, including all electrical damping, enclosure air compliance, et cetera, in addition to spider and surround.

I can see why that could be a little confusing, but the speaker is ultimately a mechanical device, and it has a mechanical Q. The only unusual part is that the speakers mechanical Q is also affected by the electrical things we connect to the voice coil!

Quote:
Originally Posted by chris661 View Post
With regards to output impedances, valve amps are usually a couple of ohms.
Got any references you can point me at?

Also, are we talking triode outputs with heavy negative feedback, or pentode outputs with no feedback? I don't see any way that the latter can be brought down to just a few ohms.

In response to my quick calculation (same one I showed here), someone else ran an LTSpice simulation, and also came up with output impedances in the hundreds of ohms. No negative feedback, pentode output, and, of course, results only as good as whatever LTSpice valve model he happened to use.

I've been too lazy to sit down and measure this, but I guess I should just do that and be done with it.

-Gnobuddy

Last edited by Gnobuddy; 27th October 2017 at 03:30 AM.
  Reply With Quote
Old 27th October 2017, 10:57 AM   #24
chris661 is online now chris661  United Kingdom
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Sure, here's a little reading. Haven't heard of anything past 10ohm.

Typical output impedance for tube amps

Chris
  Reply With Quote
Old 27th October 2017, 05:14 PM   #25
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by chris661 View Post
Sure, here's a little reading. Haven't heard of anything past 10ohm.

Typical output impedance for tube amps

Chris
Thanks for the link!

The first post states in that thread states "...typical output impedance of a push-pull and a SET amp..", so he's talking about triode output stages.

I know nothing about the strange and bizarre world of triode audio power amps, so I Googled to find a valve used in them, and found a reference to a 2A3 triode. I then found a 2A3 datasheet that lists an 800 ohm anode resistance (ra, or rp if you prefer "plate"). The same datasheet also recommends a 2500 ohm load.

Let's re-do my quick calculation using ra=800, and a transformer that steps down the impedance from 2500 ohms to 8 ohms.

The transformer impedance ratio is (2500/8), or 312.5:1.

An 800 ohm anode resistance will therefore be stepped down to (800/312.5), or 2.56 ohms. So, without any negative feedback, we can expect a single-ended triode power amp using a 2A3 to have an output impedance in the ballpark of 3 ohms.

Applying 10 dB of negative feedback around the output stage would reduce that by a factor of roughly three, and you would end up with roughly one ohm. Exactly as Chris661 says.

So it seems nothing is fundamentally wrong with my calculation method; we have now verified that it does in fact predict about the right output impedance for a triode output amp.

And now we also know that the same calculation method, applied to a 6V6 beam tetrode instead of a triode, estimates a 300 ohm output impedance with no negative feedback, and maybe 100 ohms with 10 dB of negative feedback. Output impedance is about a hundred times higher than for the triode.

A look back at the two datasheets shows there is no mystery about this: ra for the triode was specified in the data sheet at 800 ohms. ra for the 6V6 beam tetrode was specified at 77,000 ohms, very nearly one hundred times larger than for the triode. And, sure enough, my quick calculation estimated the pentode/beam tetrode amps output impedance at about a hundred times larger than the triode amp.

I think I have a better idea now why people are willing to put up with the high cost, abysmal efficiency and minuscule output power of triode power amps. It's the only way to get any sort of reasonable damping factor from a valve power amp!

But, going back to the "tube bass guitar amp" comment in this thread that triggered this tangent, I have never heard of a triode-output valve bass guitar amp, for obvious reasons. The few that became popular always used pentodes or beam tetrodes in the output, so we now have reason to believe that they were effectively driving their speakers from a constant current source (Zo of amp >> speaker voice coil resistance Re)

-Gnobuddy
  Reply With Quote
Old 27th October 2017, 08:35 PM   #26
freddi is offline freddi  United States
diyAudio Member
 
freddi's Avatar
 
Join Date: Aug 2005
Vented box for bass guitar design - HELP!
a lot of classic tube push pull pentode output amps (Fender, Marshall, Traynor, Mesa), used a cathode coupled inverter with loop feedback from the secondary's tap to the cathode circuit of the inverter. For economy , reliability and simplicity, the 2nd plate load resistor was ~20% higher than the first for balance. (if a really 'long" tail were employed such as CCS, then the plate loads would be equal).

using 420v plate supply and 12at7, such an inverter should swing 50vrms with 150K grid resistors on the outputs. Mesa used a 12ax7 on their Strategy 400. (not bad in hifi application but I've only run it with Klipschorns - it's output Z might help with some overdamped stuff.

not a lot of nfb available with that scheme - guess that's good with avoiding oscillation. My SRW10 (made by Eminence) have qt ~0.23.

RE OHMS 4.33 FS HZ 52.22
LE MH .74 MMS GMS 25.60
QM 2.94 CMS mm/N .3622
QE .250 RMS NS/M 2.8583
QT .230 VAS LTRS 68.19
XMAX MM 3.00 SD SCM 366.10
BL TM 11.99 EBP 206.2
EFF % 3.70 SPL dB 97.7


these old tube amps had a more fluid midrange in general than the solid state amps of their times - how much of that were HF rolloffs and higher output Z = ?

YBA-3 power section
https://i.imgur.com/c4wCBeP.jpg

Last edited by freddi; 27th October 2017 at 08:40 PM.
  Reply With Quote
Old 28th October 2017, 12:29 AM   #27
freddi is offline freddi  United States
diyAudio Member
 
freddi's Avatar
 
Join Date: Aug 2005
Vented box for bass guitar design - HELP!
here is a page dedicated to analysis of a typical simple push pull EL34 guitar amp with cathode - coupled inverter with negtive feedback to its cathode circuit from a tap on the output transformer's secondary and typical feedback resistors of ~5K in inverter's tail, 100K from the transformer's secondary.

the calculated output Z is from 4 to 5.2 ohm or so with 5K & 100K (depending upon secondary tap. This would drop further if there were another gain stage included in the feedback loop and more nfb applied.


Designing for Global Negative Feedback
  Reply With Quote
Old 28th October 2017, 01:27 AM   #28
freddi is offline freddi  United States
diyAudio Member
 
freddi's Avatar
 
Join Date: Aug 2005
Vented box for bass guitar design - HELP!
note in Aiken's discussion of pentodes

Effective Output Impedance

Using the formula for output impedance, along with the originally calculated open-loop gain of 41, and assuming a feedback resistor, Rf, of 100k, and an input resistor, Ri, of 5K, and an internal output impedance of 16 ohms, the closed-loop effective output impedance would be:
Zout = ((Ri + Rf) * Ro) / (Ri + Rf + Ro + Ri*A)
= (5K + 100K) * 16 / (5K + 100K + 16 + 5K*41)
= 5.2 ohms

The Effect of Changes in Load Impedance

Note that in a tube amp, the load impedance greatly affects the open-loop gain, because the internal plate resistance of the typical pentodes is very high, so the effective output impedance would be rather large if you didn't have a load connected. The impedance seen looking into the output would be equal to the effective plate resistance of the tubes divided by the impedance ratio of the tubes. When a load is connected, it reflects back an impedance equal to its value multiplied by the impedance ratio of the transformer. This means that the effective internal output impedance is equal to the output load in parallel with the tube plate resistance reflected to the secondary. It is still fairly close to the load resistance, because the plate resistance of a typical pentode is quite large.
What this all means is that the open-loop gain is going to change when a different load impedance is connected to the same tap. This change in open-loop gain changes the effective output impedance and the overall closed-loop gain of the amplifier.


I think there a few tube amps running ultralinear such as one Sunn which was very close to Hafler's Dyna MK!!/III. That achieved good damping factor with nfb and the pendtode gain/ split load triode inverter (6an8, 7199). Audio Research's D90 pentode amp with regulated screen supply was listed as having a damping factor of "12".
  Reply With Quote
Old 28th October 2017, 03:28 AM   #29
Gnobuddy is offline Gnobuddy  Canada
diyAudio Member
 
Join Date: Mar 2016
Quote:
Originally Posted by freddi View Post
note in Aiken's discussion of pentodes
Aiken's website was also where I got the 10 dB number, which he suggests is about the maximum amount of negative feedback found in valve guitar amps.

How does he arrive at the stated "internal output impedance of 16 ohms", do you know?

-Gnobuddy
  Reply With Quote
Old 28th October 2017, 04:22 AM   #30
PB2 is online now PB2  United States
diyAudio Member
 
PB2's Avatar
 
Join Date: Sep 2004
Location: North East
Gnobuddy your calculation is off.

The free air Q's become Qec, Qmc, and Qtc in a closed box.
Right out of T&S theory.
  Reply With Quote

Reply


Vented box for bass guitar design - HELP!Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Please critique my first compressor design (for bass guitar) vitahamin Instruments and Amps 0 2nd September 2013 09:58 AM
vertical line cabinet design for bass guitar tapehead ted Instruments and Amps 25 29th August 2012 01:18 AM
Guitar/Bass-Guitar Loudspeakers used as Bass-Midrange for Home Audio Applications tiefbassuebertr Multi-Way 2 23rd April 2012 05:40 PM
small preamp design for a bass guitar Gene_Johnson Instruments and Amps 42 25th November 2009 05:12 PM
Bass guitar amp design guidelines Bill622 Instruments and Amps 12 1st December 2008 10:25 PM


New To Site? Need Help?

All times are GMT. The time now is 03:32 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio
Wiki