High impedance out amps VS as low as possible. - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 2002, 01:19 PM   #1
diyAudio Member
 
Join Date: Mar 2002
Location: Montreal, Quebec, Canada
Default High impedance out amps VS as low as possible.

I've been thinking about the output impedance of audio amps.

Typically, the goal is to make it as low as possible, but what would happen to the sound if I would double the voltage capability of the amp, but have an output impedance of 8 Ohm. Or, let’s go a little more extreme. Multiply the amp voltage by 10, have an output series resistance of 72 Ohm. With an 80 Ohm load, I can make a super low distortion amp is the 50 watt range.

As the series resistance increases, to the speaker, the signal should look more like a current regulated feed VS a voltage regulated feed.

I got this idea from the way video is driven. The driver is usually 2Vp-p, but with a 75 ohm resistor in series. With the 75 ohm load on the cable, the resulting video is 1Vp-p.

Brian.
__________________
_______
Brian
  Reply With Quote
Old 25th October 2002, 01:49 PM   #2
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Output impedance

The two major reasons to have a low output impedance is to improve damping of the woofer and to make sure that the electrical response at the speaker terminals is flat over the frequency band.
The impedance of a speaker is not uniform over the frequency band barring a few exceptions. So if you put a large impedance in series with it ( the output impedance ) the voltage from the amplifier will divide itself depending on the impedance at various frequencies. So a flat output at the amp will no longer be flat at the speaker terminals which will in turn lead to alteration of the frequency response of the speaker.
So identical amplifiers with different ( significant) output impedance should sound different with the same speakers. We are assuming that the speaker is like most systems with a varying impedance over the frequency range.

As far as low output impedance is concerned for damping purposes: The speaker's internal resistance itself is a limiting factor. So for the output impedance to have practically no significant effect it should be about less than 1/10 of the speakers resistance. At least that is what I have heard many people say. I myself think that if it is less than 0. 2 ohms it should be fine. I could make out a very slight difference with 0.5 and 0.1 ohms . It probably depends on other factors also. A 0.2 ohms output impedance with a Dynaudio Audience 50 speaker shows significant ups and downs in the frequency response at the speaker terminals.

You cannot compare this with RF. I am not a RF person but I thought that the equal impedance there is for maximum power transfer.
Cheers.
__________________
AM
  Reply With Quote
Old 25th October 2002, 01:56 PM   #3
diyAudio Member
 
Join Date: Mar 2002
Location: Montreal, Quebec, Canada
Thanks for the well explained answer.

When it comes to really high fidelity equiptment, what would be the typical output impedance.
__________________
_______
Brian
  Reply With Quote
Old 25th October 2002, 02:03 PM   #4
diyAudio Member
 
Join Date: Aug 2002
Location: NY
Default Re: Output impedance

Quote:
Originally posted by ashok

You cannot compare this with RF. I am not a RF person but I thought that the equal impedance there is for maximum power transfer.
Cheers.
When working with RF, or any high enough frequency, matching impedances is a way to minimize reflection on the line. I say "high enough frequency", because it's the frequency of the speaker relative to how long the cable is, so it would be entirely possible to have this become an issue at audio frequencies.

When you're dealing with a long cable, and a high frequency, you run into the transmission line effect. Basically, if you send out a signal, you will have some reflected signal coming back if the impedances aren't perfectly matched. The magnitude of the reflected signal is directly related to the impedance of the line and the impedance at the end of the cable that it reaches. This is bad because you can have a lot of stuff on the line interfering with your signal, but it's also bad because it can require your supply to do wierd things, like drive your full output into a short, even though you have the cable and a load connected.
  Reply With Quote
Old 25th October 2002, 02:12 PM   #5
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Optimum Z out ?

Hi Brian,
I had the same problem when I built my hybrid valve amp with no feedback. The output impedance was very high at around 1 ohm and it sounded far inferior to the Audiolab 8000 amp with an output impedance of much less than 0.1 ohm.
Not only was the bass not tight enough but the balance in the upper frequencies was wrong. I added feedback and dropped this to about 0.2 ohms and things improved . With an additional pair of output transistors and some more feedback this came down to 0.1 ohms or so and the system now sounded very nice. But I must add in spite of the problems, in the no feedback mode
the midrange was really nice . With feedback it was still nice but something had changed and it was not as nice as the no feedback mode.

So low feedback with plenty of output transistors ( low output impedance) should sound good !
I would suggest an output impedance that is less than 1/20th of the lowest speaker impedance that you plan to use. For a 4 ohm driver that would be less than 0.2 ohms.
I have also found that the equivalent amount of series or shunt feedback sounds different. So go ahead and test your design. It is the only way you can find out how it will sound.

Cheers.

In commercial designs you will find output impedances that are far less than 0.1 ohms. They could be as low as 0.01 ohms. In valve based systems this could be much higher. I am not sure but 0.5 ohms may be quite common. Some valve amps beat the pants off solid state designs !!
__________________
AM
  Reply With Quote
Old 25th October 2002, 02:33 PM   #6
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Transmission lines

Yes I forgot about the transmission line and reflections. I thought that this was only at very high frequencies and and not at audio frequencies. In fact this is a big issue in the 'cable' arena. Some say it is silly to talk about this in the audio frequency range and others who make expensive cable say it is not. Making exotic cable produces cables with different inductance and capacitance which react differently with different systems. In addition I have heard of a high-end amp break down due to rf oscillation when it was used with an 'exotic cable'. If you hear differences it could be just 'different and possibly likeable' but not necessarily accurate like an ideal wire. This difference would most likely be due to the same effects as 'output impedance'. Maybe someone should make identical systems in which one has the amp mounted on the speaker with not more than 1 foot of normal heavy guage wire. This test should be revealing.
Can someone work out the reflection in a speaker cable
(3 meters long ) with a 1 or 5 Khz square wave and confirm if it will affect the audio signal?
Cheers.
__________________
AM
  Reply With Quote
Old 25th October 2002, 02:49 PM   #7
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Default Re: Output impedance

Quote:
Originally posted by ashok
So for the output impedance to have practically no significant effect it should be about less than 1/10 of the speakers resistance. At least that is what I have heard many people say.
For some tube (solid state also) amps this can be true, rather high output impedance. This can colouring the sound with certian speakers, bass reflex in particular (Am I right here?).

(My answer got a little time delayed...)
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 25th October 2002, 03:01 PM   #8
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Default Re: Transmission lines

Quote:
Originally posted by ashok
Can someone work out the reflection in a speaker cable
(3 meters long ) with a 1 or 5 Khz square wave and confirm if it will affect the audio signal?
Cheers.
This tranmission line talk is correct in theory but in real life? Minor importance I would say.

The speed of the signal in the speaker cable is 0.6-0.7 times the speed of light. How long time does it take for a reflexion to get back to the amp in a 10 meter (33 ft) cable? 55 ns! Which frequency does this correspond to? 18 MHz.

Can this tranmission line talk make any sence for audio signals?
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 25th October 2002, 03:05 PM   #9
diyAudio Member
 
traderbam's Avatar
 
Join Date: Jan 2002
Location: Earth
I agree with most of what's been said. I'd say <0.5-ohms is quite adequate for most speakers.
Line reflections are not important for at least two reasons. Firstly the wavelengths at audio frequencies are massive. Secondly the output impedances and input impedances of equipment and speakers are radically mismatched anyhow. Cables sound different because of distortion effects and the effects of reactive loading on the circuits driving them. Some power amps, like Naim's, get very upset with high C low L speaker cables.
  Reply With Quote
Old 25th October 2002, 03:18 PM   #10
diyAudio Member
 
Join Date: Aug 2002
Location: NY
Default Re: Re: Transmission lines

Quote:
Originally posted by peranders


This tranmission line talk is correct in theory but in real life? Minor importance I would say.

The speed of the signal in the speaker cable is 0.6-0.7 times the speed of light. How long time does it take for a reflexion to get back to the amp in a 10 meter (33 ft) cable? 55 ns! Which frequency does this correspond to? 18 MHz.

Can this tranmission line talk make any sence for audio signals?
The speed has nothing to do with it, it's the wavelength. I haven't done any math with this stuff in a while (I'm used to 50-ohm outputs, 50-ohm cables, and 50-ohm loads), but I think you follow the 1/10th rule. So for RF frequencies, you're talking like a meter. For audio frequencies, you're talking more like 1500 meters.

So if your audio cable is more than a mile, it becomes an issue.

(somebody check my math, like I said, been a while)
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Impedance ratio and T-amps Sasquatch Class D 3 14th April 2013 06:44 PM
Tuning 604 Project - High Q and High Output Impedance? Seadweller Multi-Way 20 18th February 2008 07:49 PM
I coined the phrase HIGH VIVIDITY, to differentiate our amps from High Fidelity ones vax9000 Tubes / Valves 8 31st October 2006 01:10 AM
High Impedance GC GregGC Chip Amps 10 10th March 2004 01:51 PM
[Q] Why high impedance? Richard Moon Multi-Way 2 29th July 2002 11:18 PM


New To Site? Need Help?

All times are GMT. The time now is 07:14 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2