Frequency compensation for JX92S

[audiobomber]

I'm likely going to build the 8L JX92 monitors shown on the Jordan site. I see there's a circuit suggested for frequency compensation. I assume this is BSC?

I'm not crazy about putting a resistor and inductor in series with the woofer. Can this circuit be implemented at line level instead?

Dan

 

[Dave Jones]

I'm likely going to build the 8L JX92 monitors shown on the Jordan site. I see there's a circuit suggested for frequency compensation. I assume this is BSC?

I'm not crazy about putting a resistor and inductor in series with the woofer. Can this circuit be implemented at line level instead?

Dan

I have just built a pair of Jx92S monitors in sealed 8 liter boxes. The speakers are near a wall. The recommended BSC with the 4 Ohm resistor is too much. It sounds best with 1 Ohm, but quite acceptable with no BSC at all.

Why are you not crazy? 🙂 Don't worry that the passive filter will degrade the sound.

 

[audiobomber]

Re: Re: Frequency compensation for JX92S

I have just built a pair of Jx92S monitors in sealed 8 liter boxes.

Cool, how do you like them?

Why are you not crazy? 🙂 Don't worry that the passive filter will degrade the sound.

Having just gone from a passive to an active crossover in my main system, I have fresh experience of what an inductor does to the sound. And a resistor is going to eat up the amp's damping factor. If I can implement a BSC circuit between the amp and preamp, I'm sure it will sound better.

OTOH, like you said, maybe I don't need frequency compensation at all.

 

[Dave Jones]

Re: Re: Re: Frequency compensation for JX92S

Cool, how do you like them?



Having just gone from a passive to an active crossover in my main system, I have fresh experience of what an inductor does to the sound. And a resistor is going to eat up the amp's damping factor. If I can implement a BSC circuit between the amp and preamp, I'm sure it will sound better.

OTOH, like you said, maybe I don't need frequency compensation at all.

What do you mean "what an inductor does to the sound"? It does what it's supposed to do. Doesn't it?

Concerning damping factor, remember that the inductor effectively takes the resistor out of the circuit at frequencies where the impedance is very high. Again, I wouldn't worry about it.

I reiterate, when the speakers are near a wall, it sounds best (to me) with a 1 Ohm resistor in parallel with the 1.5mH inductor - better than without the BSC circuit. Your ears may vary.

I skipped a question. How do I like them? Let's put it this way: I slept with them last night, and I still respected them this morning.

 

[Stocker]

an inductor in the line level signal in an active preamp will have different effect than a similar value inductor in line to a speaker.

 

[Josephjcole]

I'm likely going to build the 8L JX92 monitors shown on the Jordan site. I see there's a circuit suggested for frequency compensation. I assume this is BSC?

I'm not crazy about putting a resistor and inductor in series with the woofer. Can this circuit be implemented at line level instead?

Dan

If you look halfway into this post there is message explaining a BSC ciruit done line level for a fostex 167.
Joe

this post

 

[audiobomber]

Re: Re: Re: Re: Frequency compensation for JX92S

What do you mean "what an inductor does to the sound"?

An inductor is a filter. It resists AC voltage changes. This causes the bass to sound fatter and slower and less dynamic.


Concerning damping factor, remember that the inductor effectively takes the resistor out of the circuit at frequencies where the impedance is very high.

The problem of decreased damping factor worsens as frequency decreases (woofer motion increases at lower frequencies, more momentum is harder to stop when signal stops.)

I reiterate, when the speakers are near a wall, it sounds best (to me) with a 1 Ohm resistor in parallel with the 1.5mH inductor - better than without the BSC circuit.

I don't doubt that. I still think a line-level BSC is a better way to go.

I slept with them last night, and I still respected them this morning.

😀

 

[audiobomber]

Re: Re: Frequency compensation for JX92S

If you look halfway into this post there is message explaining a BSC ciruit done line level for a fostex 167.

That's a good start, thanks Joe. Too bad they didn't give a formula.

 

[planet10]

An active BSC circuit

http://www.t-linespeakers.org/tech/bafflestep/bstepcompo.html

and a clipping from Paul Joppa (posted a couple places -- this is from the Full Range Forum)

Line level equalizer for baffle step (long)

[ Full-Range Driver Forum ]

Written by Paul Joppa at 20 Apr 2004 22:52:44:

A little while ago, Jon Ver Halen asked me to look into a line level equalizer that would provide the same equalization as the popular LR and LCR speaker level filters that are often used for single-driver speakers. Bass reflex and to some extent TQWT designs seem to be especially common applications. This started as a commercial project, but it turned out to be so simple that I thought it would be better to just put it in the public domain. Here is the result:

An externally hosted image should be here but it was not working when we last tested it.

In the midband the gain is reduced to R2/R1+R2, and is -6dB when R1 = R2. C2 provides the low frequency boost by restoring the gain to unity at the lowest frequencies. C1 is optional, and provides some treble boost (restores the gain to unity at the highest frequencies).

The LF time constants are R2*C2 and (R1+R2)*C2. The HF time constants are R1*C1 and (R1 || R2)*C1, where R1 || R2 means R1 in parallel with R2. Frequency is 1/(2*pi*time constant).

To make the calculations simpler, I assumed that the source impedance (Rsource) is much lower than R1 or R2, the load impedance (Rload) is much higher than R1 or R2, and the capacitance of the load (Cload) is much smaller than C1, which is itself much smaller than C2.

In practice, as long as Rsource is less than half of R1 || R2, and Rload is more than twice R1 + R2, the response will be within a dB of the predicted values. The greatest practical difficulty is with Cload. It must be less than 10% of C1 in order for it to affect the performance by less than 1dB. Most of its effect is above the HF corner frequencies, so it might be reasonable to use a Cload as high as 20% of C1. If C1 is not used, then the only limitation is the (R1 || R2)*Cload time constant, which will cause a treble rolloff.

As an example, I determined parameters for two implementations (one active, one passive) to match Martin King's revised speaker level circuit, shown at:

http://www.quarter-wave.com/Project04/Project04.html

His speaker-level circuit has approximately 5.2dB midband cut, with low frequency corners at 220 and 400Hz and high frequency corners at 10kHz and 18kHz.

The active implementation assumes a tube gain stage to make up the 5dB loss, and a direct-coupled cathode follower so that Cload is very small and Rload is essentially infinite. Here are the values:

Rsource = 15k
R1 = 220k
C1 = 68pF
R2 = 270k
C2 = 0.0015uF
Cload = 10pF

I compared this using the PSpice circuit simulation program to a model of King's circuit, assuming a constant 8 ohm speaker impedance (roughly what he measured, above 200Hz). The match is good, except this circuit is 1dB down at 20kHz due to the 10pF input capacitance of the cathode follower.

For a passive implementation between preamp and power amp, the Cload would be larger. Most tube amps have around 100pF input capacitance, and the cable will have more. A frequently used number is 1000pF for long cables, but with short cables of 3 feet or less, a Cload of 200pF might be achievable. Then you could use the following values:

Rsource < 2k ohm
R1 = 10k
C1 = 1500pF
R2 = 12k
C2 = 0.033uF
Rload > 50k
Cload = 200pF

A minimum load of 12k is presented to the source, which is on the low side but achievable with most gear; in the midband the load is 22k.

For experimenters, a useful modification would be make R1 a variable resistance, or replace both R1 and R2 with a linear taper potentiometer. This way the magnitude of the boost can be adjusted, to compensate for room acoustics for instance.

I hope this is useful to the community.

dave

 

[audiobomber]

An active BSC circuit

http://www.t-linespeakers.org/tech/bafflestep/bstepcompo.html

and a clipping from Paul Joppa (posted a couple places -- this is from the Full Range Forum)
dave

That's the ticket! Thanks Dave.
And a big thank you to Paul for his generosity.

Dan

 

[audiobomber]

Re: Re: Re: Re: Frequency compensation for JX92S

Concerning damping factor, remember that the inductor effectively takes the resistor out of the circuit at frequencies where the impedance is very high.

Sorry, just ignore what I said about this in my other response. I was misreading what you said.

I agree the resistor is taken out of the circuit at low frequencies, but it will still compromise the amp's control of the mid and high frequencies, and it will waste power (lower efficiency). A line-level circuit won't.

I've been playing around with the components in my speaker crossovers a lot, and anything I've inserted between a driver and the power amp is a compromise. Between the amp and preamp, if it's done right, is much less intrusive.

 

[DIAR]

I prefer my GM MLTL without any BSC. I tried resistor (various) and choke (1,5 mH) combo but it killed the sound. Adding a good quality (4,7 uF) capacitor parallel with resistor and choke helped much but not enough.

 

[MJK]

An inductor is a filter. It resists AC voltage changes. This causes the bass to sound fatter and slower and less dynamic.

The problem of decreased damping factor worsens as frequency decreases (woofer motion increases at lower frequencies, more momentum is harder to stop when signal stops.)

A couple of comments on the statements above :

1. Lets assume that the baffle step for a reasonably sized cabinet occurs at 400 Hz. Depending on the actual width of the enclosure this frequency might be slightly higher or lower, but this frequency is a reasonable starting place for discussion.

2. The inductor in the BSC circuit is sized to become active at 400 Hz and then a resistor is placed in parallel to pad down the mid range and high end, above this frequency, the desired dB amount to rebalance the SPL response.

3. Below 100 Hz all of the signal passes throught the inductor, it is equivalent to an additional length of speaker cable and contributes only a small additional DC resistance. Considering the high impedance of the driver/enclosure system below 100 Hz, it should be obvious that the inductor has no impact on the bass performance. The driver output and motion is not changed for a given input signal, turning up the volume will obviously increase the driver motion.

4. Above 400 Hz, the impedance of the inductor is rising and all of the signal transitions to pass through the parallel resistor. The SPL of the driver is shelved down to be consistent with the bass output from the driver/enclosure system. In fact, adding a BSC circuit corrects a local phase shift that arises due to the baffle step phenominon.

Conclusion :

A BSC circuit should have no effect on a speakers bass performance or an amps damping factor unless it has been incorrectly sized and implemented. If for arguements sake, we assume that there is some small degradation to the signal from the BSC circuit my experience is that the improvements in overall system SPL performance are so large that this assumed degradation is not significant. If your BSC is resulting in a "fatter and slower and less dynamic" performance the problem is in the midrange response, not the bass response, which would lead me to believe that the parallel resistor value is too large.

Hope that helps,

 

[audiobomber]

3. Below 100 Hz all of the signal passes throught the inductor, it is equivalent to an additional length of speaker cable and contributes only a small additional DC resistance.

4. Above 400 Hz, the impedance of the inductor is rising and all of the signal transitions to pass through the parallel resistor.

I agree with you on everything you said Martin, with an exception. IME an inductor has more effect on the sound than its small DCR would indicate. I have heard my speakers with an inductor in the circuit and without (replaced by an active cct in front of the amp) and there is an easily noticeable difference in speed, detail and dynamics, all in favour of the active circuit. Same goes for the resistor on my tweeter. It noticeably degrades performance vs a line-level XO.

If your BSC is resulting in a "fatter and slower and less dynamic" performance the problem is in the midrange response, not the bass response, which would lead me to believe that the parallel resistor value is too large.

My comments on the effects of an inductor and resistor between the amp and the driver are based on my experiences with an active vs. a passive crossover, not on a BSC cct. Nevertheless, I firmly believe that speaker-level components (caps, resistors, inductors) are not as transparent as line-level components. And this was apparent even when I used top quality parts in the passive crossover.

I certainly don't doubt your statement that BSC benefits outweigh the losses. All I'm saying, is that a line-level beats a speaker-level circuit for sound quality and efficiency.

Dan

 

[MJK]

A crossover and a BSC are two completely different animals with totally different functions. In my variable BSC design, I can completely remove the circuit from the system and then gradually bring it back with the twist of a dial. Powered by a SS amp, I do not hear any degradation in the sound or the transient performance of the speaker driver. The only thing heard is the gradual rebalancing of the SPL response resulting in a significant performance improvement. This is true of the Lowther drivers I use which I believe would quickly reveal any sonic degradation resulting from the BSC circuit.

 

[morbo]

MJK, sorry for the slight veer off-topic, but when you say:

A crossover and a BSC are two completely different animals with totally different functions. In my variable BSC design, I can completely remove the circuit from the system and then gradually bring it back with the twist of a dial.

what about when you crossover right at the BSC frequency at a slope close to the BS drop off rate? Is this what you mean by your variable BSC design? I am very interested in how yoy accomplish what you describe with dial.

 

[MJK]

what about when you crossover right at the BSC frequency at a slope close to the BS drop off rate? Is this what you mean by your variable BSC design? I am very interested in how yoy accomplish what you describe with dial.

All of my recent speaker designs are single driver full range enclosures. If you look under the General Speaker Related Articles link on my site you will find the documentation for the variable BSC I use with my Lowther ML TL speakers.

I used a BSC on my two way Focal TL, it was placed ahead of the crossover as an add on fix after the fact. Probably not an optimal design but it worked out very well in that particular situation. You can find that schematic under my Project #1.

 

[audiobomber]

A crossover and a BSC are two completely different animals with totally different functions. .

They're different, but they're similar too.
- they're both filters
- they both insert an inductor between the amp and LF driver
- they both insert a resistor between the amp and the mid/high frequency driver

Where we disagree is on the effect of the latter two points.

 

[MJK]

Where we disagree is on the effect of the latter two points

I agree, we disagree. No point in continuing the discussion.

 

[soongsc]

I think some degradation using different components might cause sound degradation. Designs based on ideal components should be good regardless what names you give it. I have found that foil inductors preserve the liveness of music. This I think was because the phase is preserved throughout the long wiring of the inductor.