Impedance matching anyone

More like designed to match the ratio of tube supply voltage to available plate current to the loudspeaker's nominal impedance. Then, as mentioned in other posts, negative feedback (or triode finals) were used to bring the impedance of the speaker terminals down.

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Tim Wescott
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Tim Wescott
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Yes,

some Fender guitar amplifiers are designed with current feedback that actually raises their output impedance. They drive the speakers fine.

I modified one to change the feedback to voltage feedback to lower the output impedance. There was a subtle change to the bass response.

A speaker driven with a hi Z will have a more pronounced peak at the bass resonant frequency and will have a bit more treble.

The difference is a bit subtle and depending on the OPs application may not be a big issue.

Mark

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Mark

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Strangely, loudspeakers connected to toob amps with those big ol\'
output tranformers seem to honk OK, and they\'re certainly nowhere near
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Reply to
John Fields

John,

Any tube amplifier that claimed to be high fidelity had enough negative feedback to bring the output impedance below 1 Ohm. The ancient Radiotron Designers Handbook talks of damping factors of around 20.

Tam

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Tam/WB2TT

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Since the ratio of the supply voltage, V, to the plate current, I, is


      V
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Reply to
John Fields

You have already done the right thing. Passive impedance matching is certainly possible with a transformer, but there are two problems. I, there will not be enough power to driver the speaker, and 2, speakers are not designed to be matched - they need a zero impedance voltage source.

d

Pearce Consulting

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Reply to
Don Pearce

On Tue, 17 May 2005 11:14:13 GMT, snipped-for-privacy@pearce.uk.com (Don Pearce) wroth:

Speakers will work just fine when connected to a wide range of source impedances.

Speakers (drivers) in a two-way system, for instance, seldom "see" a zero impedance voltage source.

Jim

Reply to
James Meyer

Hello All,

I'm trying to impedance match two systems together. I have an audio source (600 ohms) and a speaker (15 ohms) and I need to find a way of getting them to work.

Can anyone point me in the direction of a good book or website that will explain how this might be done. I have built an op amp amplifier and this works. But if a passive approach is possible I would like to take that as my interface will be battery powered.

Many thanks for any help/advise.

Naveed

Reply to
corlioni1976

No they won't. One look at the impedance curve of a speaker should tell you that they need a low - if not zero - source impedance if they are not going to honk like geese.

Such speakers generally have no pretensions to, nor need for, fidelity.

d

Pearce Consulting

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Reply to
Don Pearce

I'm talking about the considerable mismatch of an electrically matched system - where the source impedance would be 8 ohms instead of zero. That makes for considerable unflatness in response.

I've never come across a speaker designed to be powered from anything other than as low an impedance as possible - can you point me at one? In fact I didn't even know it was an electromechanical possibility.

d

Pearce Consulting

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Reply to
Don Pearce

source

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Hi Don

Many thanks for your reply.

Naveed

Reply to
corlioni1976

I'm not talking about amplifier design, but speaker design. And of course guitar amplifier designers strive for the sound they want, not fidelity.

As I said.

I believe his application is to monitor what is happening on a line out, therefore we must, as best guess, consider that fidelity is a prerequisite.

d

Pearce Consulting

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Don Pearce

In the 1930s, when lowish impedance triode and high impedance pentode output stages were used - both without feedback - the loudspeaker was selected to *sound right* on a particular amplifier, although high impedance pentodes always produced a prononunced bass resonance.

My copy of the 1940 Radiotron Designer's Handbook talks about the better performance of speakers with low impedance drive as produced by triodes or negative feedback. The issue is damping of the bass resonant frequency. The moving mass resonates with the spider spring compliance and the voice coil and magnet motor when shorted provides significant damping. The energy absorbed by the motor is dissipated in the resistance of the voice coil.

High impedance drive maintains current at higher frequencies where voice coil inductance would otherwise reduce current, and so lifts treble.

At the end of the 1930s, the 0.1% THD figure became attainable using voltage feedback; and along with the low distortion, output impedance fell to much less than the loudspeaker impedance, and true voltage drive became standard, rather than just a favourable damping factor.

Since then, quality loudspeaker systems which are sold without matched amplifiers, are always designed for zero source impedance. I believe that some powered speakers have interesting feedback producing non-zero and even negative output impedances.

The tuning of bass reflex systems is badly affected by wrong drive impedance, and sealed systems boom badly because the bass resonance is much less damped. You can easily pick the difference - feed an audio gen into an amplifier, connect a speaker and sweep the gen from 200Hz down to below the bass rolloff. Now insert a 20 or 50 ohm resistor in series with the speaker, readjust the volume, and sweep again. Typically, the high-Z drive gives an enormous bass peak at around the

+20db level ! You still get quite a peak even with an 8 ohm resistor.

Another experiment is to get a loudspeaker system and tap on the woofer, listening to the *boomp* sound, with the terminals open and short circuited and 8 ohms connected. The extra damping is very evident.

If you do impedance match, you will still hear sound, and if this is not a hi-fi application, the sound can be clear and usable.

Reply to
Roger

Yes, but you were wrong. The plate resistance of a tube is defined as the slope of the V/I curve at the operating point; for a 6L6 class A amplifier operating at 350V plate voltage and 60mA plate current the plate resistance is 33k ohm. The specified _load_ resistance in this case is 4.2k ohm.

Correct, and that'll make sure that you run out of current at the same time you run out of voltage. It _won't_ make the amplifier impedance equal to 8 ohms however -- with a 33k plate impedance and a 500:1 impedance ratio the impedance at the amplifer terminals will be around

66 ohms.

That all depends on how the amp is designed, doesn't it? A 2A3 final stage would have a load resistance of 2500k at the plates for about a

300:1 impedance transformation ratio. A 2A3 in a class-A amp has an 800 ohm plate resistance, so the amplifier as a whole would have a terminal resistance of about 2.7 ohms. Not zero, but much less than 8 if my math skills haven't abandoned me. Similar results could be had with a pin-toad amplifier using negative feedback.

------------------------------------------- Tim Wescott Wescott Design Services

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Tim Wescott

Did any of them use the ultra-linear connection in the final? That's a non-obvious form of feedback.

I would class guitar and your Hammond completly differently than a hi-fi amp: in the case of of the amp being part of the instrument you can tailor the whole system to include the speaker response; in the case of the hi-fi amp you would have to be more careful about how the sound is reproduced.

"Roger" mentioned on an adjoining thread that with a high-impedance amplifier output you need to match the speaker to the amplifier. IIRC the cool early hi-fi speakers (and possibly current, I just listen to boom boxes) were "low efficiency", presumably with sacrifices made in the speaker to get better sound. If you're dinking around with the speakers at the same time you're dinking around with the amplifier it's hard to tell what changes are affecting what...

------------------------------------------- Tim Wescott Wescott Design Services

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Reply to
Tim Wescott

You don't need a perfect impedance match to transfer most of the available power. An 800 ohm to 8 ohm transformer rated for 400mW (42TU008) is probably close enough and costs about $2 from Mouser

Reply to
John Popelish

Hello Tim,

Only one out of several tube amps I repaired many moons ago had feedback. The biggest ones didn't, yet they sounded really cool.

Same with our Hammond organ which came to us as a basket case. So I had to get schematics and dive in. Same thing, no feedback, yet fidelity is really crucial on those instruments.

Regards, Joerg

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Joerg

Well, a 16 Ohm speaker is not going to work very well when driven by a 600 Ohm source. ;-(

--Mac

Reply to
Mac

On Wed, 18 May 2005 05:06:09 GMT, Mac wroth:

That's why the subject line of this thread includes the words "impedance matching".

A 16 ohm speaker driven from a 600 ohm source *will* work fine. It will simply have less output energy at a fixed voltage from the 600 ohm source than when a 600 to 16 ohm matching transformer is used between the source and speaker.

A zero ohm source impedance such as Don recommends is *very* difficult to achieve with passive components. The original question asked about transformers. A zero ohm impedance winding on a transformer will result in zero volts of output.

Jim

Reply to
James Meyer

I think they refer to these devices as audio amplifiers.

A passive approach will work only if you don't need any gain.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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