| astrodog |
I am building a Silicon Chip kit with the LM3876 chips - I'd lke to use a pair of car speakers (I have spares) if possible. An addition in the kit mentions using supply rails at +/- 28V for this.
If I use an 18+18 VAC transformer - should this give me the +/- 28V.
Given these speakers will be in my observatory - quite small, used late at night - so quite low volume - should using this chip with 4-ohm loads be a problem??
TIA
Lee |
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| Nuuk |
| That should be fine Lee. |
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| AndrewT |
| the 18Vac transformer should be about right. |
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| astrodog |
| Thanks for the advice guys - my plan, at least initially is to use my iPod or laptop as source - should I expect to have to use a preamp?? |
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| Nuuk |
Expect the possibility! ;)
I don't wish to be offensive but you say that you are going to play MP3 files into car speakers, so I don't know if quality is an issue here. If not, you shouldn't need a pre. |
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| astrodog |
| If I could reliably pick a 256kbps MP3 from CD I wouldn't play them! I'll see how it all sounds and tinker from there..... |
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| astrodog |
Music!!
My amp (one channel so far) produced some music! Stoked! Small amount of hum also - although so far I've not bothered with trying to minimise this with star grounding etc.
It sounded surprisingly good, considering I had the amp board connected to a bare, cheapo, paper-cone woofer alone, without any crossover/filter....
The hum I had completely disappeared upon shorting the inputs, or connecting an ipod - is this hum then due to ground loops?? |
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| AndrewT |
| quote: | Originally posted by astrodog
The hum I had completely disappeared upon shorting the inputs, or connecting an ipod - is this hum then due to ground loops?? |
was the input open circuit when the hum was detectable? |
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| astrodog |
| Yes - hum only heard when input open circuit. |
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| AndrewT |
open circuit input is not a normal operating condition.
Ignore this. |
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| astrodog |
Cool.... :D
Hopefully by this time tomorrow I'll have a stereo amp! Even if the heatsink is currently held to the base board mock-up by cable ties.....
Thanks for the advice so far..... |
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| astrodog |
But.... (and I have searched!).....
I now have two working channels - using an iPod headphone out as source, I can adjust volume easily past where I would generally listen, but a line level source (CD player) is insanely loud.
Where/how do I control the volume?? <ducks>
I know a dual linear pot comes into play - do I just insert in the input lines??
Is this where I need a preamp??
I did pick up a "Universal Stereo Preamplifier Kit" (another Silicon Chip kit based on LM833 opamp) - it says "can be constructed for use with a magnetic cartridge, cassette deck or dynamic microphone..."
Did I get the wrong thing??? I'd just like to be able to boost a lower level input if required - or decrease the volume of a loud source......
Hope this isn't TOO basic...... :D |
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| AndrewT |
Hi,
use a stereo log law pot to adjust the volume.
This makes a passive pot and impedances/resistances must be carefully chosen to get good performance. |
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| Nordic |
about 10k works for the majority of GCs.
you can also make a fake law pot by useing a 100k linear pot and a resistor.. a little googleing is in order...
I doubt you will get good performance with that preamp... look on the site for the pedja rogic buffer there is a pedja rogic buffer for dummies thread... that shows how to wire one up with some stripboard.. |
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| astrodog |
I think I can wire up one of those..... thanks for the info.
A question: Doing a search for 2SK170 at fibra brandt brings this up:
Stock Database
Part Number Description Price
Found:5
2SK170-TOS N-FET 40V 20mA 0.4W 0.49EUR
2SK170-BL/TOS 0.70EUR
2SK170-GR/TOS ask
2SK170-BL / TOS 2X MATCHED 2.20EUR
2SK170V-TOS 2.09EU
Does it matter which I were to use - I don't understand the differences between these????
TIA, Lee |
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| justblair |
You need the BL version for the buffer. The two letter suffix referrs to the spec of the Jfet in terms or current handling I think. (My memory is fried with 2sk170 specs, I have been working on this buffer myself for two many hours)
The 2xmatched I suspect means that you are gettng two of them that have been tested and are a matched pair. There is a fairly high tolerance on the specs of this Jfet if you check the datasheet.
Matching the Jfets yourself means buying up a bundle of them, testing them and grouping them. The idea being you want the two channels matched with identical (or nearly anyway) specced components.
That will be why a premium is being charged.
I bought my bundle of 50 from someone in the trading post here a while ago. I paid a lot less for them than those prices and will be matching them up myself (this weekend hopefully if the gf will stop making plans for me, got a Kilt fitting already)
Instructions for matching them are available on t'internet, Its a slow process, but relaxing kinda. |
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