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Old 4th September 2007, 01:34 AM   #1
Paiku is offline Paiku  Canada
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Default Fine-tuning enclosure - questions

Hello all,

I'm working on a speaker cabinet for my bass guitar rig (Traynor tube amp, 130W "clean", but those are "tube watts", so... ). I'm not new to loudspeaker design, and I've read the basic texts, but I find they don't go deep enough.

About the "final tuning" of a ported enclosure; how should it be done, and why is it so important?...

1. Of course, a driver's actual T-S specs can vary from what the manufacturer publishes. So you have to tune your enclosure to your actual driver, not your theoretical one.

But, in your modeling software (i'm using WinISD), you can adjust the port length away from "optimal" and see the results. The speaker's response changes a bit, but not catastrophically so. It doesn't even look noticeable.

What if your driver's Fs or Vas, for example, is higher than you expected. Are you tuning to MATCH the actual value, or to COMPENSATE for it?

All those admonitions to fine-tune your enclosure imply that your project will sound like an AM radio in a windstorm, if you don't. Is there more to this than frequency response? Is it about dispersion (group delay)? Is it about matching the enclosure Fb to the driver's resonance? What if you don't?

2. That's the "why." Second question: "How?" What should the impedence curve look like when the driver and box are properly tuned?

Any help is appreciated! Thanks all,
-John

Project: cabinet for bass guitar. It's a 1x15". Driver: Eminence Legend CB15 (Fs 34Hz). Design goals: flat response down to 41Hz... well, as flat as practically possible. Low group delay (<<25ms). Power handling 130Wroot mean square (sine). Ported alignment: 5.6 cu.ft. (160 litres) tuned to 34Hz.
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Old 4th September 2007, 03:05 AM   #2
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Further, the T/S parameters aren't really a single number, but a curve and change depending on how hard you are pushing them....

dave
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Old 4th September 2007, 03:28 AM   #3
GM is offline GM  United States
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Greets!

Basic T/S speaker design programs assumes a zero output impedance and very small signal to the speaker and why minor changes to the vent length makes little/no difference, ergo if driven with a typical SS amp and large enough wire, then tune it to whatever you want or do as most folks do and settle for just being in the ball-park based on a sim.

Tube amps OTOH usually do have enough output impedance to raise the driver's effective Qts enough to matter, ergo the cab's Qtb (vented) or Qtc (sealed). Since tube amps are typically impedance matching devices, tuning a vented alignment to Fs yields the widest LF gain BW and why fine tuning is considered critical. It's tuned when the impedance peaks on either side of Fs are equal in amplitude.

GM
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Old 4th September 2007, 04:22 PM   #4
Paiku is offline Paiku  Canada
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Thanks GM, very interesting!

"Widest low-freq gain bandwidth". So, the amp is not going to have as much gain at the freq's where the speaker impedence peaks? It's been a long time since analog electronics class...

In WinISD Pro, I raised the Qts of the driver, to see the effect on transfer function, impedence, etc. I dunno if that's a valid model of what will really happen in a tube-amp set-up, but the results were interesting...
- much deeper bass! -3dB point extended from 53Hs to 43Hz. At 41Hz (low-E), response is +1dB vs unmodified curve.
- group delay peaks at 20Hz instead of 30Hz. Group delay at 41Hz is 10ms instead of 11ms, but rolls off slower as freq rises. Not a big deal.
- impedence curve - no change - interesting.
- cone excursion peak is 7mm (~50Hz) vs 5.5mm, before. Xlim is 9.8mm, so I'm not too worried. Infrasonic cone excursion is greatly increased!!!

Thoughts?

Anyway, I'll tune those impedence peaks as close to equal as I can. And I'll order that subsonic filter I've had my eye on...!

Thanks,
-John
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Old 4th September 2007, 10:39 PM   #5
Paiku is offline Paiku  Canada
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Interestinger and interestinger...

Ok, with a little extra-curricular surfing today, I've learned about the link between output impedence and electrical damping. Now I'm using WinISD's "Series Resistance" field. Interesting results.

Not sure what the output impedence ("damping factor") of my tube amp is, but if it's in the range of 1-2 ohms, I should still be okay. A little more deep-bass response, not much change in cone excursion or group delay.

If I'd known this, I woulda bought a lower Q driver to go with my tube amp.

Other comments?
Thanks,
-John
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Old 5th September 2007, 04:34 PM   #6
GM is offline GM  United States
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Default Re: Fine-tuning enclosure - questions

Greets!

Remember the old adage 'as impedance grows, so goes the power'? Tube amps are impedance matching devices, so need a unity damping factor (DF = 1) for the most efficient transfer of power, ergo the higher the driver's impedance at Fs and inductance rise up high, the less power transferred. IOW, in the driver's nominally 8 ohm/whatever BW the frequency response (FR) ~mirrors the impedance plot and elsewhere is boosted proportionately, so a puny 1.0 W SET amp may perform as if it's a much more powerful one around the box tuning and up high if it has a high inductance.

SS amps OTOH have a vanishingly low output impedance, so the driver's Qts combined with the cab's Qtb or Qtc defines its FR down low and its inductance defines its HF roll off rather than the SET's HF boost response.

AFAIK there's no speaker design software that can sim a tube amp's effect on the driver/cab response, only mimic it to some extent by adding either series resistance and/or a baffle step compensation ckt. (BSC) which reduces the effective system sensitivity Vs the SET's ability to ~keep it the same as the driver's.

You can calc its DF:

Connect a high watt 4, 8, 16 ohm resistor (R Load) to the output of your amp depending on the tap used. Feed your amp with a ~50 Hz signal. Set the volume to a high level, but don't let your amp clip. Measure the output voltage across the load resistor. Remove the resistor and measure the output voltage again. It should be higher now. Don't leave the amp on very long with no load or damage to the amp may occur, and to you if a tube grenades!

then:

Amp resistance = R load*(V without load - V with load)/V with load

DF = R load/amp resistance

GM
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Old 5th September 2007, 05:14 PM   #7
sreten is offline sreten  United Kingdom
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Default Re: Re: Fine-tuning enclosure - questions

Quote:
Originally posted by GM
Greets!

Remember the old adage 'as impedance grows, so goes the power'?
Tube amps are impedance matching devices, so need a unity
damping factor (DF = 1) for the most efficient transfer of power .....

GM
Errrr......

I'm sorry but this is completely wrong, DF should never = 1, (for hi-fi).
They have taps to match valve and speaker currents, that is what matters.

DF = 1 applies to all theoretical amplifiers with unlimited current.

Valve amplifier transformers saturate in the bass with current and with
very few exceptions produce very low power into a too low impedance.

The valve amplifiers recommended load should be used.

/sreten.
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Old 5th September 2007, 05:25 PM   #8
sreten is offline sreten  United Kingdom
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Default regarding fine tuning


Hi,

A well designed box is fairly tolerant of speakers variation. If the box
volume is say less than 50% of nominal Vas, then any variations of
Fs and Vas - one goes up and the other goes down, is not critical.

A port frequency of 34Hz means the box is tuned for a 5 string.
For a 4 string with low E or D around 40Hz is better.

The ouput impedance and damping factor depends on the resistance
of the transformer windings and the amount of feedback applied to
the output stage - it can be quite high with low feedback.

As said this effectively increases Qts and changes tonality.

/sreten.

note : a subsonic filter is not needed for bass guitar only use.
The valve amplifier will also effectively throttle back low
frequencies by distorting - this is part of their sound.
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Old 5th September 2007, 08:43 PM   #9
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Default Re: Re: Re: Fine-tuning enclosure - questions

Quote:
Originally posted by sreten
I'm sorry but this is completely wrong, DF should never = 1, (for hi-fi).
They have taps to match valve and speaker currents, that is what matters.

DF = 1 applies to all theoretical amplifiers with unlimited current.
You are a bit mired in the "all amps should be constant voltage devices".

I started to be lead out of this when i had a chat with one of the Western Electric design engineers. He said that an amplifiers output impedance should be equal to the speaker's impedance (ie damping factor = 1). These were the state of the art amplifiers in the middle of the last century.

Nelson's F1 has a damping factor >1, the F2 (and many zero feedback SE tube amplifiers) have DF close to 1. Nelson has some good articles on the subject at FirstWatt.

This just means that the speaker needs to have good self-damping (ie a speaker should never be considered separately from its partner amplifier -- they are a system.

The speaker si have been designing of late are designed for & work best with amps that have higher output impedance.

dave
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Old 5th September 2007, 08:47 PM   #10
Paiku is offline Paiku  Canada
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Thanks guys, endlessly fascinating stuff.

Last night I made measurements similar to what you (GM) described, but using two different resistors, a 10 and a 33 Ohm (I wasn't comfy running my amp unloaded for any length of time!). I came out with a source impedence of 14.8 Ohms. Seems kinda high to me. But then, the V's didn't quite add up to the IR's, so perhaps there was some significant inductance or capacitance going on. I was testing at 200Hz.

DF=1 for the most efficient transfer of power, I think that's right. Impedence matching, source resistance equal to load. If the load is less, then most of the voltage drop occurs in the amp; if the load is greater, then current drops off.

Anyway, this isn't hi-fi, it's a bass guitar amp & cabinet. A little distortion is alright (expected), but I would like even frequency response. The amp in question is a Traynor YBA-3, which has four EL34's (push-pull) in the output stage, and a huge tranny. (More info on YBA-3 ). There is no choice of taps for different load impedences. In fact, the documentation of the day was a bit vague about what load was best to use. The amp was often sold with two 8-ohm 4x10 cabs! The amp has two speaker jacks which are IN SERIES (unusual).

Sreten, I was planning on tuning to 34Hz because that's the driver's Fs. GM explained that this will maximize the system's LF bw gain. Tuning to 42 Hz does give me a little more SPL (1dB) between 40Hz and 80Hz (good), but also increases group delay (by 0-2ms) in that range (bad - transients lose definition).

Guys, thanks for all your feedback on this project! I'm planning on building the cab this weekend, so must settle on the internal volume, soon. Any more comments/discussion, I'm all ears!

-John
PS: can create subsonics with a bass guitar by thumping on the strings. But if the amp won't pass those tones, so much the better.
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