Transistor Headphpone Amp - Page 1
Forum section: electronics / beginners

author: Amoebius
author_rank: Contributor
timestamp: January 23, 2023, 08:23:30 pm
content: Hi,I made a somewhat crude Headphone Amp for my Smartphone and my Beyerdynamics DT 990 Pro 250 Ohm Headphones.I am an absolute beginner and want to ask you if it is safe for my devices to use it. Or do i have to add components?Greetings,DirkP.S. I use a 9V batterie for power.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2023, 08:46:23 pm
content: The output transistors could do with some emitter resistors, otherwise they might go into thermal runaway and smoke. D1 and D2 should be smaller diodes, such as the 1N4148. There will be some crossover distortion, which can be reduced by adding a resistor in series with the diodes.

author: Benta
author_rank: Super Contributor
timestamp: January 23, 2023, 09:27:27 pm
content: Yes, it's safe to use and will not harm your signal source. Crossover distortion will be high, your bias circuit is not good.I won't comment on the schematic artwork.

author: Amoebius
author_rank: Contributor
timestamp: January 23, 2023, 09:47:02 pm
content: Ok, thank you for the answers.Why is the bias not good? When i simulate it, i get a clean sinus wave.What did i do wrong? I want to learn.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2023, 10:09:21 pm
content: Quote from: Amoebius on January 23, 2023, 09:47:02 pmThe simulation doesn't represent reality.The base-emitter voltage on the transistors will fall, as they heat up, which results in more base current, causing them to turn on more. This eventually leads to them overheating.There will be crossover distortion, when one transistor stops conducting and the other starts.

author: Benta
author_rank: Super Contributor
timestamp: January 23, 2023, 10:23:02 pm
content: Quote from: Amoebius on January 23, 2023, 09:47:02 pmTransistors and diodes are not ideal and not linear. The myth of VFor VBEbeing 0.7 V you can forget.The goal is setting a stable DC quiescent current through your output transistors. For this, you first need to minimize the tolerance, temperature dependence, and non-linearity of VBE.Solution: emitter resistors; @Zero999 already mentioned this.If emitter crurrent increases, so will the voltage drop over the resistors, thus incresing the base voltage at the transistors. Result: a stable DC operating point.For this to work, a stable bias voltage between the the two bases of your output transistors is needed. Your two diodes will not provide that, they also have temperature dependency, tolerance, and non-linearity.That'sthe difficult part. I'll leave to you think about it.

author: Amoebius
author_rank: Contributor
timestamp: January 24, 2023, 06:11:19 am
content: Ok, thank you both.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 24, 2023, 06:55:55 am
content: Bravo.Its a very good start.I look forwar to seeing more.

author: JoeyG
author_rank: Regular Contributor
timestamp: January 24, 2023, 06:58:37 am
content: Can you attach the ASC file for us to look at?

author: Amoebius
author_rank: Contributor
timestamp: January 24, 2023, 07:44:06 am
content: Quote from: JoeyG on January 24, 2023, 06:58:37 amI did some changes and added 2 Ohm Emitter Resistors.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 24, 2023, 10:46:57 am
content: The schematic in the OP is not a good choice for many reasons.For expensive headphones like yours, I'll get a dedicated IC. TPA6120A2 has outstanding performance and can drive high impedance headphones, too:https://www.ti.com/product/TPA6120A2.If you have the skills to make the PCB, buy only the chip, it is in stock at Mouser for less than $5. There are ready assembled devboards from TI, though they are more expensive. I bet there are ready made modules with TPA6120A2 from other sources, too, but I didn't search.There are other examples with TPA6120A2 as a headphones amp only, apart from the examples in the datasheet (you don't need all that, that is an example of how to use the IC with a differential DAC)https://www.ti.com/lit/ds/symlink/tpa6120a2.pdf. It's hard to beat that level of performance.One example is in their evaluation board:https://www.ti.com/lit/ug/slou394/slou394.pdfhttps://www.ti.com/lit/ug/slou169/slou169.pdfThere are many more projects online (not from TI) if you search for headphones amplifier with TPA6120.

author: Whales
author_rank: Super Contributor
timestamp: January 24, 2023, 11:23:18 am
content: Nice simple schematic.I have a habit of making mistakes when calculating my bias resistors for transistors. Sometimes I get lazy and throw a trimpot in so I can fix the problem whilst listening. This also has the benefit of allowing you to hear what the distortion sounds like if the bias is bad.(I once made a few stages of amplification with 2n7002 transistors that used some red LEDs as a ~2V gate bias voltage. It worked better than I expected)Emitter resistors are indeed a good idea. Your headphones are very high impedance (250ohm) so they won't have much detriment at 2ohm (or probably even 10ohms?).Please tell us if you get it working and how it soundsI wouldn't worry about improving it any more until after you build and test it. This also lets you keep it as something to compare other things you build against, regardless if they sound better or unexpectedly worse.

author: jasonRF
author_rank: Regular Contributor
timestamp: January 24, 2023, 11:32:25 am
content: Quote from: Amoebius on January 24, 2023, 07:44:06 amIf I did my back-of-the-envelope calculation correctly, 2 Ohms is borderline for thermal stability. I would be tempted to start with 4.7 Ohms. (This calculation assumed that the biasing diodes are thermally isolated from the output transistors, which seems reasonable for a typical implementation of this kind of circuit).You will learn something by building your circuit (EDIT: and I agree with Whales that you should build it and listen to it), but I would expect it to have significant distortion. Attached is a slightly modified version of your .asc file, where all I did was specify the diodes, and change the simulation commands (modified .tran, and added .four) so that it automatically calculates the distortion for you. I am assuming you are running LTSpice. Before running it, click on the 'simulate' tab, open the control panel, and in the compression tab make sure no boxes are checked. Once you have done that you simply run simulation. After it is done, under the 'view' tab, open the spice error log. It will show you the THD%. I get that your circuit has something like 0.5% distortion, which will not sound good. The best way to reduce distortion is to use feedback.You can definitely build a fully discrete headphone amplifier/buffer that has low distortion and sounds good, but dedicated chips designed for this purpose will have better overall measured performance. They can control and match devices on a chip in ways that you can never do with discretes, plus they have experts with significant resources working on the designs and most of us have nowhere near the expertise that they have. Which approach you take depends on your goals. When I build headphone amplifiers, it is 100% for fun, and I personally find discrete design to be more fun that using chips. But that is a personal preference.Jason

author: jasonRF
author_rank: Regular Contributor
timestamp: January 24, 2023, 01:34:41 pm
content: In case you are interested, the thermal calculation for the emitter resistor goes like this.Assume that the bias voltage across the diodes is independent of the temperature of the output transistors, as this will be the worst-case scenario. There will be some equilibrium operating point where there will be a bias current through the output stage and the output transistors will be at some temperature.Now think about what happens if the temperature of the output transistor junctions each increase by an amount $$\delta T_0$$ This will cause the base-emitter voltages to each decrease by approximately 2.2 mV per degree C (this is a rough rule-of-thumb). Since the bias voltage across the diodes is staying the same, this extra voltage appears across the emitter resistors, increasing the bias current in the output stage by an amount $$\delta I_0 = 0.0022\, \delta T_0 / R_e$$ This increased bias current will increase the power dissipated by each transistor by about $$\delta P_0 \approx V_{CE} \, \delta I_0 \approx .01\, \delta T_0/R_e$$ where we have let $$V_{CE} \approx 4.5\, Volts$$ (ignores the voltage drop across the emitter resistors) since you have 9V supply. The increased power dissipation will then further increase the temperature of the transistor junctions by about 200 degrees per Watt (from the datasheet for the to-92 package), which can be written $$\delta T_1 \approx 200\, \delta P_0 \approx 2\, \delta T_0 / R_e$$ So the initial bump in temperature of $$\delta T_0$$ has resulted in an additional bump in temperature of $$\delta T_1$$ This in turn will result in further increases in temperature. In order for this not to blow up and melt the transistors ("thermal runaway"), you will need $$\delta T_1 <\delta T_0$$ which for your circuit means $$R_e > 2$$ Ohms.While this is a rough estimate, it should get the job done. I have made 9V battery powered amps using 3.3 Ohm emitter resistors and they worked fine. The last one I made had 4.7 Ohm resistors.jason

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 24, 2023, 03:35:09 pm
content: Safe to use question: you’ll hear a pop when turn on the battery power. Your headphone is on the expensive side, might want to protect it from the full 9V transient with a relay by disconnecting before DC levels settle.Designing and simulating it can be a good exercise.

author: Amoebius
author_rank: Contributor
timestamp: January 24, 2023, 05:51:48 pm
content: I built it on a breadboard, with 2 Ohm Resistors, it did sound ok. Not great. I listened to some music and to a 1kHz sine, i couldn`t hear heavy distortion.I measured 1,3 Volt over the diodes.Thank you for all the replys, the distortion measurement in LT-Spice and the thermal calculation is very helpful.When i get home, i will use 5,6 ohm resistors, i should have some.Greetings,Dirk

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2023, 06:20:47 pm
content: Connecting the return of the headphones to +V, a smaller output and larger input capacitor will help to minimise the power on pop.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 24, 2023, 07:49:41 pm
content: In this case C1 should be reversed, with the + to the headphones.

author: jasonRF
author_rank: Regular Contributor
timestamp: January 25, 2023, 12:23:22 am
content: Quote from: Amoebius on January 24, 2023, 05:51:48 pmYou are welcome. Just note that the thermal calculation is a rough approximation. 2 Ohms (or 2.2 or 1.8Ohms) might be high enough to prevent runaway - or it might not - you would need to test. But I am confident that 1 Ohm will probably be unstable and 3 Ohms should be safe.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 25, 2023, 06:17:20 am
content: If you’re not happy with the distortion you need negative feedback and higher gain. This is a good example how can the schematics improved:https://www.electroschematics.com/headphone-amplifier/

author: tszaboo
author_rank: Super Contributor
timestamp: January 25, 2023, 04:24:31 pm
content: For 9V batteries, there are a lot of circuits that are collectively called CMOY, they are easy to build and a good beginner project. Breadboard friendly, or you can even order kits with PCBs, and it's designed to go into a small mints tin can.

author: Amoebius
author_rank: Contributor
timestamp: January 25, 2023, 05:10:15 pm
content: Ok. I'll take a look. Thanks.

author: Amoebius
author_rank: Contributor
timestamp: January 26, 2023, 06:38:19 pm
content: I did a bit of reading and changed, as suggested, the gain and added negative feedback. When i simulate it with LT Spice i get a THD of around 0,04%.I the circuit right? Can i apply nfb the way i did?And what polarity should C4 have?

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2023, 06:51:06 pm
content: Quote from: Amoebius on January 26, 2023, 06:38:19 pmNegative feedback can be capacitively coupled.The polarity of the capacitor can be determined by the sign of the voltage across it. The side with the higher DC bias should be the positive. Moving the mouse pointer over the nodes, after running the simulation will tell you the DC voltage, at different parts of the circuit.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 26, 2023, 11:42:28 pm
content: You can add more loop gain by splitting R4 into two, and feed the midpoint with a cap like C4 from the output. Should lower distortion.

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2023, 10:42:09 pm
content: What are your goals?How about using an op-amp? The NE5532 might even be able to directly drive your 250 Ohm headphones hard enough, failing that, add a couple of transistors.Have you considered a transformer? If your device is designed for low impedance headphones, a 1:4 ratio transformer will make the headphones look like1/16of the impedance, so about 16 Ohms, and increase the amount of power by a factor of 16. This will obviously shorten the battery life of your device slightly. The transformer needs to be decent quality to avoid distortion.

author: Amoebius
author_rank: Contributor
timestamp: January 28, 2023, 07:06:55 am
content: Quote from: Zero999 on January 27, 2023, 10:42:09 pmI want to learn how to build a transistor amp and ways to improve it.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 28, 2023, 02:43:39 pm
content: If you like the improvements a bootstrapped R4 brings, you can think adding a difference amplifier. It would lead you to a discrete transistor opamp topology, just ashttps://community.element14.com/members-area/personalblogs/b/frank-milburn-s-blog/posts/op-amp-made-from-discrete-componentsWidth those improvements it can operate down to DC from dual rail supply. If the power supply is good enough it will give you very nice bass - who does not like solid bass?

author: BrianHG
author_rank: Super Contributor
timestamp: January 28, 2023, 04:10:39 pm
content: Quote from: Amoebius on January 28, 2023, 07:06:55 amYou can venture into single ended design territory. You might also use a mosfet in voltage follower at the output of the single ended design while using bipolar or mosfet for the constant current pull-down.