Please need help to calculate gain from circuit. - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 24, 2018, 07:31:34 am
content: Can anyone help calculating this amplifier circuit's gain?I do not know if this task are a insane large one or even manageable. For me it is totally impossible.You have properly seen this circuit before, that Is what I constantly are bothering you with! :-)

author: DuPe
author_rank: Regular Contributor
timestamp: June 24, 2018, 10:15:34 am
content: roughly ×5?(100k//100k) / (10k+2k).anything right of level shifter are emitter followers, having gain of about 1

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 24, 2018, 10:29:45 am
content: not more then 5?! wow that was not much!!Does that mean that a i.e. CD-player with a line out at 1Vpp only end in 5Vpp to the speaker?!

author: orbanp
author_rank: Regular Contributor
timestamp: June 24, 2018, 11:53:38 am
content: Quote from: DuPe on June 24, 2018, 10:15:34 amIt is more than that!The 10k resistor is bypassed by the 100uF // 100nF caps!So the gain is (100k//100k) / 2k = 25Peter

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 24, 2018, 12:37:39 pm
content: Thanks a lot, so it is about 25Vpp out pr channel?Does that not sound as about 36W out?EDIT: the transformer are 26V-0V-0V-26V on the schematic, it is about 80W out, why?

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2018, 01:07:48 pm
content: Yes, 25 sounds about right.Quote from: FriedMule on June 24, 2018, 12:37:39 pmWhat's the load impedance?For a sine wave, the RMS power is equal to half the peak power, so with a 4Ohm load: P = V2/(2R) = 252/(2*4) = 625/8 = 78W.A 26V transformer will output 36V, after losses in the rectifier, so 25V peak out seems reasonable.I doubt it'll be able to output full power continuously, as the smoothing capacitors aren't big enough, but the output transistors will probably overheat too, at that power level. It's also not a problem, since peak power occurs typically less than 10% of the time, even at full volume.

author: b_force
author_rank: Super Contributor
timestamp: June 24, 2018, 01:25:26 pm
content: Quote from: Hero999 on June 24, 2018, 01:07:48 pmEhm, well better is the use to divide the rail voltage (which is the peak voltage of the sine wave) by the square root of two (= 1.41).Which is obviously the same as the Rms value of the AC voltage (minus some losses), so in this case 26Vrms.(26^2)/8 = 85W @ 8ohm or 170W @ 4ohm (the rail voltage is 26 * 1.41 = 36.7V)However, with Class-AB amplifiers it's not uncommon to give a lot more headroom in the rail voltage.The efficiency of a AB stage is about 60-70% at most, so that means that transistors will dissipate around 50W @ 4ohm.That's quite a lot, so that makes me think, including the low amount of capacitive storage, this amp is suitable for 8ohm or higher.

author: DuPe
author_rank: Regular Contributor
timestamp: June 24, 2018, 02:00:08 pm
content: you are completely right. I missed the u and did read 2x100n instead.quite likly I need new glasses

author: FriedMule
author_rank: Frequent Contributor
timestamp: June 24, 2018, 02:21:27 pm
content: You are all so great, thanks!Wikipedia tells me that 25Vpp = Nominal level, VRMS 0.316 x 25 = 7.9VRMS.That does not sound as a lot:https://en.wikipedia.org/wiki/Line_levelThat leads me to think that 36.7V is a lot more then needed with headroom and so on.Is it possible to make it happy with 4Ohm speakers also?How many volts shall the transformator have?

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2018, 02:54:04 pm
content: The gain is 25 at high frequencies and 4 at low frequencies with a high pass cutoff of about 160Hz as shown but one of the capacitors bypassing the 10k resistor is not shown. My guess is that it is suppose to be 1uF making the high pass cutoff 16Hz.Noise for this design is high because of emitter degeneration of the differential input stage which was necessary for high full power bandwidth; this is common with discrete power amplifiers but it could be avoided if an operational amplifier was used in place of the input differential pair. Distortion performance is poor because feedback does not come from the output.

author: Audioguru
author_rank: Super Contributor
timestamp: June 24, 2018, 05:07:06 pm
content: I think it will sound awful with a lot of odd harmonics distortion since the output emitter-followers are not inside the negative feedback loop. Some crossover distortion also might be noticeable.Why not make a "normal high fidelity amplifier circuit" instead, or use an LM3886 IC?

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2018, 11:06:45 pm
content: Quote from: b_force on June 24, 2018, 01:25:26 pmThe original poster said the output voltage is 25V peak, so the output power will be 78W into a 4Ω load or half that into an 8Ω load, irrespective of the supply voltage.

author: b_force
author_rank: Super Contributor
timestamp: June 25, 2018, 12:20:28 am
content: Quote from: Hero999 on June 24, 2018, 11:06:45 pmWell, he said a 26V transformer.But anyway, it doesn't matter.Nothing is wrong here, I only wanted to point out that it helps to calculate the voltage, instead of dividing the power by two.Both methods obviously work, but if someone is going to bridge an amplifier for example, it shows better that de power goes with a factor of two.