How to calculate impedance on preamplifier - Page 1
Forum section: electronics / beginners

author: kris2ff3r
author_rank: Contributor
timestamp: July 29, 2017, 04:25:17 pm
content: HelloCan anyone help me figure out the output impedance for this preamplifier?Math ain't my strong suit.I would really like to know the input impedance as well..Please help me!See attachedThe tubes are 6SN7

author: Lightages
author_rank: Unknown
timestamp: July 29, 2017, 04:35:29 pm
content: It looks pretty simple to me, the 470K resistor on the input should be the dominant factor, so 470K.

author: kris2ff3r
author_rank: Contributor
timestamp: July 29, 2017, 04:39:57 pm
content: But isn't the input impedance of a line level amplifier usually 47k?

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2017, 04:56:22 pm
content: Quote from: kris2ff3r on July 29, 2017, 04:39:57 pmDepends on the amplifier. This one clearly has a 470k resistor between the grid any 0V which dominates the input impedance.

author: kris2ff3r
author_rank: Contributor
timestamp: July 29, 2017, 04:59:21 pm
content: Thank you for your answer.I see I've got a lot of questions.. but could I change this to 47kohms, without further modifications?And how about the output impedance? Is it harder to calculate?

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2017, 05:28:06 pm
content: Quote from: kris2ff3r on July 29, 2017, 04:59:21 pmYou could but why on earth would you do that? The higher the input impedance the better.

author: Benta
author_rank: Super Contributor
timestamp: July 29, 2017, 05:30:34 pm
content: Quote from: kris2ff3r on July 29, 2017, 04:39:57 pmNo.47 kohms is for grammophone moving-magnet-pickup RIAA preamplifiers (to load the pickup correctly).Original line-level impedance was 600 ohms, but this was based on a power transfer of 1 mW (hence 0 dBm, m for mW). Nobody uses this anymore (it's been redefined to voltage instead of power, 775 mV = 0 dBm), and you'll be fine with the high input impedance of your amp.Indeed, you should be happy about it.

author: retrolefty
author_rank: Super Contributor
timestamp: July 29, 2017, 05:56:59 pm
content: Funny nobody answered the first part request, determining the output impedance as well as its input impedance.

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2017, 06:22:54 pm
content: Quote from: retrolefty on July 29, 2017, 05:56:59 pmI've no prior experience with valves but would guess the output impedance is equal to R6 which is 330R.

author: Benta
author_rank: Super Contributor
timestamp: July 29, 2017, 06:23:15 pm
content: Quote from: retrolefty on July 29, 2017, 05:56:59 pmYeah, but now the input impedance is out of the way.For calculating the output impedance, the characteristics of the tubes are needed. I don't have the datasheets, perhaps someone does?

author: kris2ff3r
author_rank: Contributor
timestamp: July 29, 2017, 07:37:59 pm
content: Thank you all for the help!Yes, the tubes I'm going to use are Tung Sol 6SN7GTB.Note, there is only two tubes, the schematics are a bit weird, I didn't make 'em!See attached for the datasheet.I need an output impedance around 100 ohms

author: orolo
author_rank: Frequent Contributor
timestamp: July 29, 2017, 08:50:24 pm
content: I know almost nothing about tube amps, but the input stage looks like a bootstrapped inverting amplifier, and the output stage a constant current loaded follower. The amplifier is open loop, so it may be easy to overdrive. The output impedance is 330 ohms. If you need 100 ohms, you should make both R6 and R7 100 ohms, which would increase the quiescent current of the output stage nearer to 9-10mA. The output power would be close to 1mW, I guess (3-4mA max through 100 ohms).The circuit looks like a total waste of power. I think you can replicate this with jfets much more efficiently.

author: Lightages
author_rank: Unknown
timestamp: July 29, 2017, 10:30:06 pm
content: Quote from: retrolefty on July 29, 2017, 05:56:59 pmYes that is interesting. I would say the output impedance is 330R as suggested before.

author: kris2ff3r
author_rank: Contributor
timestamp: July 29, 2017, 10:40:13 pm
content: Quote from: Lightages on July 29, 2017, 10:30:06 pmCould easily be changed to 100ohms, just by changing the value of R6 and R7?Without changing the THD, Noise floor, and frequency respons to the worse?

author: floobydust
author_rank: Super Contributor
timestamp: July 30, 2017, 02:18:24 am
content: The input stage is SRPP and Rin=R3 470k.The output stage is anAkido White cathode followerorAkido push-pulllooks more like the circuit. I don't know much about it, to do exact math on Zout.It does have low Z output can drive 300R headphones.But your capacitor C3 0.22uF is too small for even medium impedance at low frequencies. Rolls off even into a 10k line load at 72Hz. I would go 4.7-10uF at least.There are lots of resources on the audio forums or at that URL.