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Author Topic: What EXACTLY is DDT Compression?  (Read 12997 times)

Dave Potter

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What EXACTLY is DDT Compression?
« on: March 19, 2005, 05:34:34 am »

I've had is on my TNT130 Bass amp for years.  I know that Peavey still use it.  Its there to stop the amp clipping and a little yellow light comes on.  Does anyone know what it is actually doing and how?  
For instance,  Is it a limiter haveing zero gain above a threshold?  A soft knee compressor? hard knee?  Does anyone know the gain ratio?

As a bassist and also running a little PV8.5C, I'd like to know. Does the little light mean "this is where the amp starts to mellow out Twisted Evil . bring it on" or "Warning! don't push me. Back off  Shocked "
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John Roberts {JR}

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Re: What EXACTLY is DDT Compression?
« Reply #1 on: March 19, 2005, 07:50:28 am »

DDT is PV's flavor of clip limiting. I guess it would be characterized as a fast attack/fast release above threshold limiter, but the threshold part is not like an outboard comp/limiter. DDT actually detects when the amplifier output diverges from what it should be based upon it's input. DDT is most commonly triggered by running out of power supply voltage. One clever part about actually detecting real clipping is that it limits at different voltages for 4 ohm or 8 ohm loads reflecting the different PS loading. A second mode that triggers DDT limiting is if the amplifier current limits which could occur well before running out of power supply voltage say you decide to parallel too many speakers. Something that even the most expensive outboard limiters can never do.

There is no single correct answer to how much DDT activity is OK. That's like asking how loud is OK. It pretty much depends on how appropriately your speakers are sized to your amplifier. The DDT time constants have been carefully selected to sound OK for a wide range of conditions but it will depend on the dynamics of your original material when it becomes "too much".

While driving your system hard into DDT will effectively compress the sound and raise the average power level it isn't as much of a power increase as hard clipping. When DDT detects clipping it folds back the gain preventing the average level from rising further also keeping the signal free of clipping distortion.

Note: a subtle point specific to DDT protected amplifiers. The general gain structure advice is to crank the amplifier gains all the way up. Since DDT has some 20 dB of gain reduction range this will provide you the most clean limited signal before running out of headroom in a preceding stage. If the input gains are potted down you will clip preceding line stages before running out of DDT protection.

Since DDT was patented years ago, you could also do a search at the PTO and read the patent which will may describe in more detail than you really want.  

JR
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Paul Magro

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Re: What EXACTLY is DDT Compression?
« Reply #2 on: March 19, 2005, 09:06:57 am »

  Back in the day when I ran Peavey amps and did not have enough amps, the DDT protection worked great.  There had been times where I ran the old CS800S with the DDT light on solid for most of the show.  The board was not sending a clipped signal.  At the end of the night, the amps were very warm, bordering on downright hot.  I was suprised that they did not thermal out.  I could hear the compression as the DDT was on.  I used the amps on subs or tops mostly.  I think it is a good feature for "hobby" bands.
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Merlin

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Re: What EXACTLY is DDT Compression?
« Reply #3 on: March 23, 2005, 03:05:32 pm »

I'm using a PV2000 as a keyboard amp now, and it runs with the DDT lights on all the time too. Sometimes it's hot enough to blister your finger at the end of a set, but it hasn't failed yet...
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John Roberts {JR}

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Re: What EXACTLY is DDT Compression?
« Reply #4 on: March 23, 2005, 03:42:02 pm »

Merlin wrote on Wed, 23 March 2005 14:05

I'm using a PV2000 as a keyboard amp now, and it runs with the DDT lights on all the time too. Sometimes it's hot enough to blister your finger at the end of a set, but it hasn't failed yet...


Yes, the power transformer is heat-sinked to the faceplate so after a few hours of max output the faceplate will be hot to the touch. Making sure it's securely racked in will help get some faceplate heat out.  Rock-on    Very Happy

JR
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Bennett Prescott

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Re: What EXACTLY is DDT Compression?
« Reply #5 on: March 23, 2005, 08:18:32 pm »

So is this just a VCA like I'm used to in my compressors? One that's triggered off the difference between the amp's input and output, but a VCA nonetheless? Or is there something else going on here?

I'm just curious because I spend about the same for a 2-channel compressor as I do for some of my 2-channel amps (I've got Crest CPX-2600s on monitors) and yet the amps claim to have transparent yet extremely effective clip limiting built in for practically no extra cost.
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Don Boomer

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Re: What EXACTLY is DDT Compression?
« Reply #6 on: March 24, 2005, 06:29:13 am »

Cool thing about DDT  (and GLC in your Crest amps) is that it follows the amp as opposed to a compressor up in front of the amp.  It is actually monitoring clipping not just a predetermined setting.  If your load changes, DDT changes ... if your line voltage changes, the DDT changes.  It's a very effective scheme
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John Roberts {JR}

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Re: What EXACTLY is DDT Compression?
« Reply #7 on: March 24, 2005, 07:10:48 am »

Campus PA wrote on Wed, 23 March 2005 19:18

So is this just a VCA like I'm used to in my compressors? One that's triggered off the difference between the amp's input and output, but a VCA nonetheless? Or is there something else going on here?

I'm just curious because I spend about the same for a 2-channel compressor as I do for some of my 2-channel amps (I've got Crest CPX-2600s on monitors) and yet the amps claim to have transparent yet extremely effective clip limiting built in for practically no extra cost.


The typical circuitry used is an OTA (operational transconductance amplifier). In general these do not have the noise floor/linearity of a professional grade VCA, but when used in the right topology, they are only effectively in circuit when the amp is limiting.

You are not going to notice .1% THD or 90dB S/N ratio when you're limiting at FULL POWER. The gain modulation due to the limiting will swamp out the THD and it's hard if not impossible to hear noise 90dB below signal.

FWIW, I used a similar topology in a 4 channel noise gate/limiter (LOFT) I designed back in the '80s, back before VCAs were widely available. IMO it worked quite well and was noticeably cleaner than a typical VCA path when running below limiting.

The OTA at my last day job had a special house part number because it was a selected part for better performance.

JR  
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Bennett Prescott

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Re: What EXACTLY is DDT Compression?
« Reply #8 on: March 24, 2005, 12:29:17 pm »

Aha! Thanks, JR.
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