Need help to calculate data for this amp. - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 14, 2018, 01:33:57 pm
content: Hi I am about to build an amplifier, one that are reasonable simple and sounds good in my ears.I have found some reviews on the amplifier but I can't figur out to calculate the data I need. I.e. gain, voltage to avoid clipping and so forth.The amplifier is 60W with a 26-0-0-26 transformer.input impedance typically 9.9k ohmsSensitivity at CD/line-level with a moderate reserve, measuring 400mV for rated output, with 510mV required for clipping.Clipping (1% THD) occurred at 74W into 8 ohms (18,7dBW).With both channels it produced 68Wpc into 8 ohms (18.35dBW), 106Wpc into 4 ohms (17.3dBW). The 4 ohm power hit 150W on short-term program-related peaks, but the amplifier don't like 2 ohms at all - the peak level was truncated by electronic limiters to just 30W effective power (8,75 dBW). Severe 4 ohms speaker loads should be avoided.Would you please help me? :-)

author: Audioguru
author_rank: Super Contributor
timestamp: June 14, 2018, 08:08:38 pm
content: Your numbers have errors:1) 60W into 8 ohms is 22V RMS which needs the power supply to produce about plus and minus 18.5VDC which is made with a transformer that is 13.8V-0V-13.8V.Correctly, you said that the 26V-0-26V transformer allows the amplifier to produce 74W into 8 ohms with a little clipping. The numbers says that the transformer makes 52V RMS which produces 147Vp-p which makes plus and minus 71.5VDC. The amplifier will have a loss of 4V p-p so the output will be 75W into 8 ohms when clipping begins.2) The output power drops when both channels produce power because the transformer is overloaded. Maybe the amplifier and transformer are rated for only 120W when the amplifier is far from clipping?60W into 8 ohms is a voltage that is 22V RMS. Your input is 400mV RMS so the gain of the amplifier is 22V/0.4V= 55 times. More gain is needed so the volume control can be turned up when the input level is low.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 14, 2018, 10:51:41 pm
content: Quote from: Audioguru on June 14, 2018, 08:08:38 pmThank you a lot, the numbers are what it says 26-0-0-26 60w out + the rest:-)Quote from: Audioguru on June 14, 2018, 08:08:38 pmI am thinking on building it as a dual mono, would 2x300VA solve the problem or shal I add two bigger transformers?Quote from: Audioguru on June 14, 2018, 08:08:38 pmAnd it clips at 0.510V RMS can you please explain the problem in more details?

author: Audioguru
author_rank: Super Contributor
timestamp: June 15, 2018, 03:31:03 am
content: How much power do you want? 60W or 74W? You will not hear a difference in loudness. Double the power sounds just a little louder.You did not say the type of amplifier so I assume it is class-AB. Then with an output of 60W its efficiency is about 60% and the heating is 40W so the transformer for one mono amplifier should be 100VA or a little more.You did not post the schematic so we do not know how much voltage loss it has. I assumed a voltage loss of 4V then a 13.8V-0-13.8V transformer is adequate.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: June 15, 2018, 05:12:39 am
content: As said, small differences in power output will be completely inaudible. As will THD below 1%.The thing which makes one power amp sound better than another is general circuit stability, especially under transients like cymbal strokes. Poor design can result in ringing or other artifacts, and that is what will result in horrible sound. Unfortunately specsheets don't tell you about that. Therefore if building an amp I'd look for a design that has had good listening reviews. More important than specs.

author: JS
author_rank: Frequent Contributor
timestamp: June 15, 2018, 05:48:49 am
content: 26+26 sounds about right for 60W into 8Ω.1% is too much for my ears, while I don't work much on audio anymore when I hear clppig I know something is about to burn in my head. That's what happens when small audio system is 5kW...An over rated power supply is one of the great factors of linear audio amplifiers. Use at least twice the capacitance your math says to get clean LF response. Or do your math for 20Hz ripple as there's where the LF consumption would be and you treat your reservoir caps as bypass for your low freq.Topology is other big deal, of course. X-over compensation should be done properly or distortion would not be nice, mainly by biasing properly Angus high open loop gain even at high frequencies. So stability becomes an issue, ringing isn't nice either. Good thing is that loads are pretty well characterized, so proper compensation can be done reasonable easy.Fast clip recover is also nice, among many other things that those puppies have.JS

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 15, 2018, 08:06:10 am
content: All I want to do, is to build this amplifier, loved the sound except a fej minor things.I know it is a non feedback type but still I miss some control of the base and some say it can clip if your line in i.e. CD is turned all the way up.The amplifier is a AB type:-)

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2018, 12:22:35 pm
content: Quote from: IanMacdonald on June 15, 2018, 05:12:39 amSorry, I couldn't disagree with you more about listening reviews. For a start they're highly subjective by their nature and are influenced by other factors such as: brand, cost, aesthetics, speakers and room acoustics, where in the case of a decent Hi-Fi amplifier, the latter two to dominate to the extent the amplifier makes no difference to the sound quality. Even then, what sounds good to one person, may not sound good to another. Personally speaking, listening reviews, even positive ones, put me off, as they often smell of audiophoolery, even if none was intended.It also depends on what the amplifier is for: Hi-Fi, guitar? Is the goal to produce pleasing distortion or audible transparency?I agree, that not all specifications such as: imperceptible noise floor and THD, mean the amplifier will be audibly transparent and not misbehave, leaving undesirable artefacts under certain circumstances, but test and measurement is the way to eliminate these, not listening tests. Also don't get too hung up on these either, as what's measurable might not be audible and the reverse is never true: test equipment is better than anyone's hearing, despite what some people wish to believe.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 16, 2018, 08:09:41 am
content: I can see that CD-output should have a standard voltage but that this standard not always get followed.My thought is that if the amplifiers gain is about 55 then would 2V in give 110V out, fare exiting the 60V out so I am missing something here:-)My goal is to avoid clipping up to maximum gain on the 60W (74W) amplifier and make up for the lack in control of the speaker.I have read about "high current" amplifier and it looks like, that they are build for control.On the other hand, I do not want to burn the components with to high current.Any ideas?

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2018, 12:40:48 pm
content: Any and all audio amplifiers produce clipping when the inputs signal level is too high for the amount of gain and the power supply voltage.I have seen the schematics of hundreds of audio amplifiers and yours is the only one with no negative feedback. Transistors are not linear so they produce lots of distortion that is reduced and eliminated with negative feedback. Negative feedback also extends the high frequency response so your amplifier cuts highs and the distortion is in the highs. Maybe you like the muffled sound when the highs are reduced.Do you know what a symmetrical sinewave with no distortion looks like? Here is a transistor with and without negative feedback:

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 16, 2018, 03:08:52 pm
content: Thanks for your reply:-)When I chose the scematic, I started out by serching for scematics from the amplifiers I did remember as the best to my ears, and luckely I found one.I have no idear how or why this shuld work, all I remember is that it was wery musical, but lacked a bit control.Other talks about that it is clipping more then meny others, it did also lack a bit in control. I think, I can always add feadback if I want to, and if I upgrade the transformer, I shuld give better control and less change for clipping :-)

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2018, 05:06:18 pm
content: Quote from: FriedMule on June 16, 2018, 03:08:52 pmWithout any negative feedback then its high frequency response is poor and it cuts distortion. Maybe you like the muffled sound.Please explain what is "control".QuoteNegative feedback reduces the gain then you will barely hear its output. The amplifier must be designed with lots of gain to use negative feedback.QuoteClipping occurs when the input level is too high and causes the output of the amplifier to try to go higher than it can go. The design of the amplifier must be improved, and the supply voltage and the supply current must be increased. If you upgrade the transformer only then this little 60W amplifier will blow up. A 600W amplifier will be a little louder with no clipping.

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2018, 05:35:20 pm
content: Quote from: FriedMule on June 15, 2018, 08:06:10 amThe circuit does have negative feedback. It's just taken before the unity gain output stage, which isn't ideal, but it doesn't mean there isn't any negative feedback at all.

author: glarsson
author_rank: Frequent Contributor
timestamp: June 16, 2018, 05:47:56 pm
content: What is the function of the two 24V zener diodes (1N4841) doing in the output stage?Why are they drawn as normal diodes? Should they be 1N4148?

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2018, 05:54:07 pm
content: I do see some negative feedback from the 100k R(blank) to the 2k R(blank). Why isn't the output stage included??

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 16, 2018, 07:33:58 pm
content: The circuit is all there is from the amplifier, wasn't aware that it should miss anything, no one have said that before.What many says about this amplifier is that it is more on the light side then the base, that you in fact should pare it with darker sounding speakers.Whar are the max current this amplifier can handle?

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 16, 2018, 09:33:58 pm
content: Audioguru sorry did forgot to reply on your question about what I mean with control.What I mean are that the amplifier shall have enough power to force the speaker to move how and when the music demands it, no struggle to start or break the movement of the speaker (iron fist).

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2018, 09:43:24 pm
content: 74W into 8 ohms is 24.3V RMS. Then the RMS current is 74W/24.3V= 3.05A. the peak current is 3.05A x 1.414= 4.3A.150W into 4 ohms for short term peaks is a peak current of 6.1A but much of the current might be distortion.The output and driver transistors have a range of ratings so some might produce more current but others might produce less current. Look at the range of ratings on their datasheets then calculate the minimum current yourself.

author: Audioguru
author_rank: Super Contributor
timestamp: June 16, 2018, 09:53:40 pm
content: You said "base" but for low audio frequencies we say "bass". You said "control" but we say "damping factor" where the amplifier has an extremely low output impedance so that it damps speaker resonances very well. It starts speaker cone movement and stops speaker cone movement when the music demands it. Some audio amplifiers have enough total negative feedback that their damping factor is more than 500 times then the output impedance is 8ohms/500= 0.016 ohms. Your amplifier has no negative feedback from its output so its output impedance is too high for a good damping factor and will sound muddy or boomy.

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 17, 2018, 03:26:18 am
content: Thank you a lot, ill try to calculate it:-)The funny thing about that amplifier is that id did never sound muddy nor boomy, it did more sound detailed in the upper Hz and lacked bass and had a bit hard time to kick when the pauke was hit and stopped.Other are talking about the amplifier, being to disckant in the sound and advices people to get more dark sounding speakers.EDIT: what I do not understand is why they claim it is a 60W when the transformer are a 300VA 26-0-0-26 with about 6A?

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 17, 2018, 03:49:34 am
content: Speaker ohm248Amp W34017085Transformer A136.53.3Transformer V262626

author: Audioguru
author_rank: Super Contributor
timestamp: June 17, 2018, 03:11:16 pm
content: Quote from: FriedMule on June 17, 2018, 03:49:34 amThere numbers are when the amplifier produces no voltage loss. But of course it has lots of voltage losses, its outputs are three emitter-followers in series.EDIT: Of course its output power is higher when it is clipping like crazy because the distortion increases the output power.