Peak to Peak output of audio amplifiers ?  - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 02:57:40 pm
content: I am planing to boost the output of ppV of one function generator, so to get more voltage out of it.Before even I test my Kenwood 2x115W RMS with it, I have to ask what is the usual voltage output in such amplifiers with MOSFet.

author: Unknown
author_rank: Unknown
timestamp: December 18, 2011, 03:09:19 pm
content: Assuming pure sine wave, about 86Vpeak to peakinto an 8 Ohmic load and about 60V into a 4 Ohmic load. Also assuming the amplifier does not attenuate at that frequency.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 03:25:15 pm
content: Quote from: Alex on December 18, 2011, 03:09:19 pmOh my, now I need a 8 Ohmic load, my amplifier has output protection circuit !!

author: Unknown
author_rank: Unknown
timestamp: December 18, 2011, 03:28:19 pm
content: Quote from: Kiriakos-GR on December 18, 2011, 03:25:15 pmDoes it cut out without a load? It doesnt need to be 8 Ohms in this case, just low enough to not trip any protection circuit. Maybe the input impedance of whatever you are powering will be adequate.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2011, 04:14:20 pm
content: It may not even be a protection circuit. Some amplifiers pass a small DC current through the speaker as part of the biasing scheme so won't work with the speaker removed.What's the lowest frequency of interest?You could connect a large choke (10mH to 10H, depending on the frequency) across the output terminals. This will allow the DC bias current through but will restrict loading at the frequency of interest.

author: don.r
author_rank: Frequent Contributor
timestamp: December 18, 2011, 04:27:42 pm
content: You can purchase dummy loads for around US$10 on eBay. $6 for a 100W and $14 for a 200W wirewound.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 04:38:16 pm
content: My amplifier is the Kenwood KA-7020 (professional grade equipment), witch has two separate amplifiers and PSU, with an output of 2x115 RMS,plus they can be connected to act as one mono 250W RMS.Frequency range : acoustical what else !About the protection circuit, if it detects lower ohms than the expected, it turns off by it self.And so its impossible to damage it.

author: don.r
author_rank: Frequent Contributor
timestamp: December 18, 2011, 04:53:11 pm
content: Quote from: Kiriakos-GR on December 18, 2011, 04:38:16 pmBipolar output stage. Common ground negative output. One relay per channel so you can test one channel at a time under a dummy load.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2011, 04:58:45 pm
content: Quote from: Kiriakos-GR on December 18, 2011, 04:38:16 pmThat's pretty normal, so you'll be fine as long as the load impedance is higher than 8R.

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2011, 05:11:13 pm
content: Quote from: Kiriakos-GR on December 18, 2011, 02:57:40 pmYou can work this out directly. Most home hi-fi speakers have an 8 ohm impedance and by Ohm's law power is voltage squared over resistance (or impedance in this case). So to deliver 115 W RMS into the 8 ohm speaker, we have:115 = V*V / 8 => V = sqrt( 115*8 ) = 30 V RMSTo determine the peak-to-peak value, we compute:Vpp = 30 * sqrt(2) * 2 = 86 VWhich is what Alex said, but this is how you work it out.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2011, 05:55:54 pm
content: Another important thing to note is that with a pure sine wave the RMS power is double the peak power. In this case the voltage initially calculated by Ohm's law was the RMS voltage because the power specified is also RMS. If you're designing an amplifier for a certain RMS output power, you need to set the peak voltage to give double the RMS power into the desired load.

author: don.r
author_rank: Frequent Contributor
timestamp: December 18, 2011, 06:19:27 pm
content: The amp in question has +-56V rails. He will never see more than 112Vpp.

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2011, 06:23:13 pm
content: Quote from: don.r on December 18, 2011, 06:19:27 pmDoes that imply you need 4 ohm speakers if you want to blow the windows out at full volume?

author: don.r
author_rank: Frequent Contributor
timestamp: December 18, 2011, 06:26:10 pm
content: Quote from: IanB on December 18, 2011, 06:23:13 pmhehe... actually its 112Vpp (fingers type too fast for brain). At 140W peak output and 100W at less than 0.1% (the 115W figure is 0.7% THD), it will never blow anything out. It only has two transistors on each side of the waveform so the current capabilities are limited. I would not run it into loads that dip below 4ohms, personally.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 06:43:54 pm
content: What about the combined mode of the two output stages, that offers 250W RMS in a single speaker?I would expect to get double output, as 2 X 86Vpp = 172Vpp (or 112Vpp under the latest info)Does it work that way?

author: Unknown
author_rank: Unknown
timestamp: December 18, 2011, 07:00:10 pm
content: Enter the new power into ians method for an idealised ballpark figure. measure the rails for a maximum plus some margin for spikes.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 07:21:33 pm
content: Thank you people for all the enlightening feedback.As soon I will manage to get in my hands the proper ohms load, I will proceed with the actual tests and measurements.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 18, 2011, 07:40:50 pm
content: Very few amplifiers have trouble with no load or high impedance loads these days. One thing to watch for is using an audio amp at full output at or above 20 kHz. Some of them just aren't fast enough and will crowbar the power supplies because the outputs can't turn off fast enough. That's usually the end of the outputs. Also, you may need protection diodes from the output to each power supply rail if you drive inductive loads like relays and motors. I've powered lots of stuff using a signal generator and audio amp. The same scheme is/was used in the big AC supplies for testing things at different international voltage and frequency values. Invertron and Elgar come to mind, but there are others.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 18, 2011, 08:19:36 pm
content: Very soon I will have an oscilloscope in my bench.I believe that with it, I will finally have a complete picture of the output, speaking about frequency response.Unfortunately it will come at the beginning of 2012, I got this confirmation today.And so I must be patient until then.