4 OHM LOAD WITH AN 8 OHM CABINET?

[eukatheude]

Isn't +3db about 1.5 times the perceived loudness?

 

[HolmeBass]

Isn't +3db about 1.5 times the perceived loudness?

It depends on the frequency spectrum and on the initial volume. My point is there are so many posts on TB that poo-poo the idea that +3 dB could ever make a meaningful difference, but that's just not true, and not my personal experience. It's not an earthshaking increase in volume, but it is clearly and plainly audible.

 

[agedhorse]

I would give my first born to gain a 3dB improvement in speaker sensitivity... that's how important it is.

A transformer would work, provided it had the proper taps. The Peavy transformer MAY be able to buck in the proper ratio, but only on the auto-former side and that should be tested before hooking up to any old amp.

 

[12bass]

I would give my first born to gain a 3dB improvement in speaker sensitivity... that's how important it is.

A transformer would work, provided it had the proper taps. The Peavy transformer MAY be able to buck in the proper ratio, but only on the auto-former side and that should be tested before hooking up to any old amp.

Thanks for posting.

What sort of losses would be involved with using such a transformer? From a quick search, the Peavey goes for $150 and weighs roughly 10 pounds. The benefit from bringing another cabinet would be considerably greater than using an impedance transformer, no?

 

[darwin-bass]

It's a Traynor SB200H and unfortunately Traynor doesn't make any output claim at 8 ohms, only 200 watts @ 4 ohms. I'd guess it's somewhere around 125+ watts @ 8 ohms. I have several 1X10 8 ohm cabinets, so adding a cab is not a problem to get to 4 ohms put I'd prefer to use only one and somehow get maximum output out of the amp. I was only wondering if there was something new on the market that I wasn't aware of to trick the amp into "seeing" 4 ohms.

Well, in theory an amp that gives 100 watts at 8 ohms will deliver 200 at 4 but that is rarely achieved in low cost amps. If that were the case you'd get 3dB more output. In the case of the Traynor, assuming 140 watts a 8 ohms and 200 at 4, you're looking 1.6 dB of more volume. Yes you can hear a 1 dB change in a steady state signal but 1.5 dB more from your amp will make NO difference.

Instead, change to a more efficient speaker. Larger surface area, higher dB output per watt or both.

 

[darwin-bass]

Isn't +3db about 1.5 times the perceived loudness?

My recollection is that 3 dB is considered the smallest change that is generally noticeable in a mix and that 10 dB is what you need to make something sound twice as loud.

My point is there are so many posts on TB that poo-poo the idea that +3 dB could ever make a meaningful difference, but that's just not true, and not my personal experience. It's not an earthshaking increase in volume, but it is clearly and plainly audible.

It is true, that a good mix is often measured in small dB changes. In studio mixing, 1 to 1.5 dB can make a difference. My experience in a live sound mixing is that changes of 1 to 1.5 dB are usually inaudible in a mix. If you want the mix to be that exacting you can't simply bump or cut by 1 dB. You have to ride the channel gain and respond to the volume ups & downs in the original source to maintain the mix. Most of us don't go to that level during a live mix.

So I would submit that for a live mix, especially where you are trying to separate vocals / keys / piano, gtrs, a 3 dB change in one can certainly make a noticeable difference in the mix for those willing to listen closely.

 

[agedhorse]

Thanks for posting.

What sort of losses would be involved with using such a transformer? From a quick search, the Peavey goes for $150 and weighs roughly 10 pounds. The benefit from bringing another cabinet would be considerably greater than using an impedance transformer, no?

I would expect that the insertion loss is maybe 1/2 dB, so other than the size/weight/cost difference of another cabinet (and the additional couple dB above and beyond the transformer solution) the second cabinet would provide more audio SPL overall.

If the size/weight/cost of the additional cabinet were a deal killer, than the transformer, or a more efficient cabinet would be the obvious choice.

Therefore the obvious choice is not as obvious as it was originally thought.

 

[Killed_by_Death]

I was only wondering if there was something new on the market that I wasn't aware of to trick the amp into "seeing" 4 ohms.

Anything that does that is just going to rob you of some power that's being output. Another cabinet will give you the maximum output.

 

[RoadRanger]

I bought mine for just over $40 shipped each via this search:
peavey automatch | eBay

 

[agedhorse]

Anything that does that is just going to rob you of some power that's being output. Another cabinet will give you the maximum output.

That's just not really true. Insertion losses are typically less than 10% on a transformer like what's being discussed. That's greater than 90% efficiency.

Another cabinet is also twice the size and weight of a single cabinet, robbing you of your ease of transport.

 

[Killed_by_Death]

Well, 10% is "some power", LOL.

 

[HolmeBass]

My recollection is that 3 dB is considered the smallest change that is generally noticeable in a mix and that 10 dB is what you need to make something sound twice as loud.



It is true, that a good mix is often measured in small dB changes. In studio mixing, 1 to 1.5 dB can make a difference. My experience in a live sound mixing is that changes of 1 to 1.5 dB are usually inaudible in a mix. If you want the mix to be that exacting you can't simply bump or cut by 1 dB. You have to ride the channel gain and respond to the volume ups & downs in the original source to maintain the mix. Most of us don't go to that level during a live mix.

So I would submit that for a live mix, especially where you are trying to separate vocals / keys / piano, gtrs, a 3 dB change in one can certainly make a noticeable difference in the mix for those willing to listen closely.

I don't have much experience mixing, but in my limited experience, once things are mostly balanced, I have found that pretty small changes in volume can be effective.

My experience is really as a bassist on stage, and as such I have found that frequently plus or minus 2 or 3 dB is extremely effective. Any more and others might feel the need to change their stage volume.

I also base this on this great page that defines a decibel and has sound samples of white noise varying by 3 dB, 1 dB, and half a dB, illustrating the actual JND. Check it out. I stand by my assertion that 3 dB is always a noticeable difference in volume, except maybe if your being utterly drowned out by something much, much louder.

dB: What is a decibel?

 

[Bassamatic]

I cannot believe the misinformation and naivety in this thread!! WOW!
1. 3dB in SPL is generally accepted as the MINIMUM change that is noticeable by people with normal hearing. In a band setting - it is INAUDIBLE!!!
2. What we really relate to is 'PERCEIVED LOUDNESS". To achieve what most people would feel is "twice as loud" is actually a 10 dB difference and takes TEN TIMES the amp power. You can test this easily enough with a smartphone dB meter if you have an amp with a wattmeter output level indicator. Put a tone through and be objective!
3. What you see when mixing is your average level, rather than peaks.
4. As correctly stated above - most amps do not double their power anyway going from 8 to 4 Ohms.

The website above is very informative, but if any of you really think you can truly hear the difference from one step to another I think you are fooling yourselves - even at 3 dB per step!

In other words - forget about fooling with the amp!

 

[12bass]

I would expect that the insertion loss is maybe 1/2 dB, so other than the size/weight/cost difference of another cabinet (and the additional couple dB above and beyond the transformer solution) the second cabinet would provide more audio SPL overall.

If the size/weight/cost of the additional cabinet were a deal killer, than the transformer, or a more efficient cabinet would be the obvious choice.

Therefore the obvious choice is not as obvious as it was originally thought.

Thanks!

Two possible scenarios:

Scenario #1: Use a transformer to convert an 8 ohm speaker to a 4 ohm load to get the full power of an amplifier

If we assume that an amplifier which puts out 500W into 4 ohms produces 350W into 8 ohms, that represents a 1.5dB difference in loudness when switching from an 8 ohm to a 4 ohm load. However, if we add a transformer to convert an 8 ohm speaker to a 4 ohm load, we see a loss of 0.5dB in the transformer. Thus, the net gain in SPL using an impedance transformer = 1.5dB - 0.5dB = 1dB total SPL increase (assuming that the driver can handle the increased power without approaching thermal and mechanical limits and suffers from no power compression).

Scenario #2: Add a second 8 ohm speaker and place it directly on top of the first, producing a 4 ohm load

If we add another 8 ohm speaker, we get a 1.5dB increase in SPL from the increased amplifier output at 4 ohms plus perhaps a few more dBs due to acoustic coupling (some literature claims 6dB).

Conclusion:

Scenario #1 produces a just audible 1dB increase in SPL (assuming the driver is linear with the increase in power), while Scenario #2 will provide an obviously higher (~6dB) SPL whilst mitigating concerns regarding thermal and mechanical limits along with power compression in the driver(s).

 

[agedhorse]

Many of the new small format amps, such as the MB-200 and Shuttle 3.0, will deliver double the power into 4 Ohm's (versus 8 Ohm's) because they are based on a regulated power supply topology rather than unregulated. That's a full 3dB.

Transformer insertion loss is a maximum of .5dB, it may be quite a bit lower.

(Relative) Power compression is usually significant only as the power level approaches the thermal/mechanical ratings of the driver. The assumption is that there is enough speaker capacity for this not to be a problem.

3dB is easily noticeable within the context of a mix.

Another alternative for a 1 cabinet solution is to change to a 4 ohm driver that works with the cabinet's tuning. This would be a better solution than a transformer in this case IMO.

 

[Foz]

...3dB is easily noticeable within the context of a mix....

3 dB is a non-trivial change in loudness in almost any context... anyone who has ever worked a calibrated sensitivity knob on a power amp or dinked a fader up or down 3 dB knows this is true.

 

[12bass]

Many of the new small format amps, such as the MB-200 and Shuttle 3.0, will deliver double the power into 4 Ohm's (versus 8 Ohm's) because they are based on a regulated power supply topology rather than unregulated. That's a full 3dB.

Transformer insertion loss is a maximum of .5dB, it may be quite a bit lower.

(Relative) Power compression is usually significant only as the power level approaches the thermal/mechanical ratings of the driver. The assumption is that there is enough speaker capacity for this not to be a problem.

3dB is easily noticeable within the context of a mix.

Another alternative for a 1 cabinet solution is to change to a 4 ohm driver that works with the cabinet's tuning. This would be a better solution than a transformer in this case IMO.

Interesting.

There are a lot of provisos here.

I was not aware that any real world amplifier can actually deliver double the power into a 4 ohm load (versus 8 ohms). Do you have any bench test data which demonstrates this? According to Bass Gear Magazine, the MB800 delivers 803W into 8 ohms and 566W into 4 ohms (continuous, 5% THD), or a factor of (0.7 x 8 ohm power) into 4 ohms, which I gathered was typical for modern class D topologies (as well as older class AB amps). So, that gives an optimal theoretical increase in SPL of slightly less than 3dB using an impedance transformer (depending on insertion loss and minimal power compression), provided the amp actually delivers twice the power into 4 ohms (which seems an exceptional case from what I gather). However, if the transformer's insertion loss is 0.5dB and the amp only delivers 1.5dB more output at 4 ohms (which appears typical), the net gain will only be 1dB.

I do not disagree that 3dB is an easily noticeable increase in SPL.

 

[agedhorse]

All of the IcePower ASC series parts feature a PWM regulated SMPS and deliver 2x the power into a 4 ohm load as into an 8ohm load.

It depends very much on the topology of the power supply, as well as the ability of the amplifier to deliver the current at the higher sustained voltage.

 

[12bass]

All of the IcePower ASC series parts feature a PWM regulated SMPS and deliver 2x the power into a 4 ohm load as into an 8ohm load.

It depends very much on the topology of the power supply, as well as the ability of the amplifier to deliver the current at the higher sustained voltage.

That is the claim, however, I've yet to see any bench test data to back it up. Bench testing of the MB800, which uses the 250ASX2, shows that it produces roughly 1.43 x the 8 ohm power into 4 ohms. Other bench tests of a wide variety of amplifiers over the past few decades show that this is a typical example. While impressive, amplifiers which can produce a doubling of power into 4 ohms appear to be a statistical outliers.

I seek to clarify this in part because I do not want people to read this thread and then assume that using a transformer to convert from 8 to 4 ohms will reliably give them a healthy 3dB increase in SPL. While such an increase appears to be theoretically possible in exceptional cases, that does not appear to be the norm from the data I have seen. Rather, in the typical case, with most existing bass amplifiers, I would expect a transformer to provide an SPL increase of between 1 and 1.5dB.

 

[agedhorse]

That is the claim, however, I've yet to see any bench test data to back it up. Bench testing of the MB800, which uses the 250ASX2, shows that it produces roughly 1.43 x the 8 ohm power into 4 ohms. Other bench tests of a wide variety of amplifiers over the past few decades show that this is a typical example. While impressive, amplifiers which can produce a doubling of power into 4 ohms appear to be a statistical outliers.

I seek to clarify this in part because I do not want people to read this thread and then assume that using a transformer to convert from 8 to 4 ohms will reliably give them a healthy 3dB increase in SPL. While such an increase appears to be theoretically possible in exceptional cases, that does not appear to be the norm from the data I have seen. Rather, in the typical case, with most existing bass amplifiers, I would expect a transformer to provide an SPL increase of between 1 and 1.5dB.

The ASX series uses a different power supply topology and is not regulated, thus my comment applies to products using the ASC series (almost all of the newly released products)

I designed a power supply that used PWM regulated voltage foldback control for high line management specifically, and this was almost 15 years ago. It's not new technology by any means.