Pics of 20Hz, 1KHz, 20KHz amplifier signal. Why is the signal shape different? - Page 1
Forum section: electronics / repair

author: olegau
author_rank: Newbie
timestamp: January 07, 2025, 01:07:45 pm
content: It seems to me that the waveform should not change from 20 to 20,000 hertz, but my Yamaha CA-700 amp shows distortion at 20 and 20,000 hertz.Who knows, maybe this is normal?

author: __greg__
author_rank: Newbie
timestamp: January 07, 2025, 01:29:23 pm
content: just use sine test signal.for square signal it is "normal" and will vary depending on settings for bass/treble knobs....

author: RoGeorge
author_rank: Super Contributor
timestamp: January 07, 2025, 01:30:14 pm
content: This forum is for writing in English only, please use a translator and post your questions in English.

author: olegau
author_rank: Newbie
timestamp: January 08, 2025, 06:56:00 am
content: Yes, I believe the amp is in good working order. The sinus signal is fain.A good site:https://sound-au.com/articles/squarewave.htm

author: Haenk
author_rank: Super Contributor
timestamp: January 08, 2025, 08:31:23 am
content: Maybe filter caps?Work through the service manual is your best option:https://elektrotanya.com/yamaha_ca-500_ca-700_amplifier.pdf/download.html

author: CaptDon
author_rank: Super Contributor
timestamp: January 08, 2025, 02:17:27 pm
content: Haenk, not a filter cap problem, why suggest? The scope shots clearly show rolloff when tested at 20HZ and 20KHz. Audio amplifiers are not designed for squarewave testing. In order to have a good representation of a 20KHz squarewave the amplifier would have to be flat out to around 200KHZ, don't see that spec very often. To show a good squarewave at 20Hz the amplifier would need flat response down to nearly D.C., don't see that spec very often either. Test with SINEWAVE please.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 03:10:14 pm
content: Quote from: CaptDon on January 08, 2025, 02:17:27 pmActually, square waves are great for testing audio amplifiers as long as you know what you are looking at and not expecting them to remain square. For example, the OPs signals all look really good

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 03:12:54 pm
content: Quote from: olegau on January 07, 2025, 01:07:45 pmYes, those look good. I assume there's a subsonic filter on or the amp doesn't go to DC.

author: olegau
author_rank: Newbie
timestamp: January 13, 2025, 12:25:02 pm
content: I tried another Yamaha CA-700. The form of the signals is similar so nothing wrong with the amp. This amp has excellent sound, similar to CA-800, CA-1000.

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2025, 12:42:43 pm
content: In the 20Hz waveform, the low frequency elements are being attenuated. I don't know what the lower cut-off is supposed to be. This might be normal, or a sign an AC coupliing capacitor is failing, since it does appear as though the lower frequencies are being attenuated a little more than I'd expect, going by the waveforms I get when putting a 20Hz square wave through different low pass filters in the LTSpice simulator.What's the setting on the bass control? Does setting it to the maximum help?What does the other channel's outout look like with a 20Hz square wave on the input?The 1kHz waveform is fine.At 20kHz, there's some slew rate distortion and loss of high frequency content. Your 20kHz square wave is not just 20kHz. It has much higher frequency harmonics: 30kHz, 40kHz etc. Your amplifier can't handle the fast rise and fall times of the square wave, hence why they show as diagonal lines on the trace and the edges are rounded. This is normal. It's amplifying the 20kHz component properly.I'll post some simulated waveforms later.

author: olegau
author_rank: Newbie
timestamp: January 13, 2025, 01:12:13 pm
content: Quote from: Zero999 on January 13, 2025, 12:42:43 pmAll tone controls are flat position.

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2025, 06:28:21 pm
content: I've simulated an amplifier with a low cut-off of 16Hz (due to C1 and R1) a gain of 10 (Avol=10) and an upper cut-off of 30kHz (GBW = F*G = 30k*10= 300k). The slew-rate is 10MV/s which should be fast enough, not to affect the result.Schematic.20Hz1kHz20kHzConclusion: the plots look similar to your oscillograph, so the channel you're measuring is working properly.The .asc LTSpice file with the frequency set to 1kHz.

author: Greybeard
author_rank: Frequent Contributor
timestamp: January 14, 2025, 11:36:11 am
content: Perfect match!fc=1/(2*pi*R*C)=1/(2*3.14*100k*100E-9)=15.9HzYour Amp ist specified 15 Hz - 50 kHz /-3dB (https://audio-database.com/YAMAHA/amp/ca-700.html).

author: olegau
author_rank: Newbie
timestamp: January 14, 2025, 12:32:19 pm
content: Thanks! I can sleep well:)