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| Need help to calculate data for this amp. |
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| FriedMule:
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 ohms Sensitivity 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? :-) |
| Audioguru:
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. |
| FriedMule:
--- Quote from: Audioguru on June 14, 2018, 08:08:38 pm ---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. --- End quote --- Thank 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 pm ---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? --- End quote --- I 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 pm ---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. --- End quote --- And it clips at 0.510V RMS can you please explain the problem in more details? |
| Audioguru:
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. |
| IanMacdonald:
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. |
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