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Impedance Matching

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Impedance Matching

Power is always reduced by running mismatched in either direction, but the tone change is different. Mismatching low (8-ohm amp into a 4-ohm cab) produces a flubbier, thicker, smoother tone; mismatching high (8-ohm into 16-ohm cab) produces a flatter, more complex, more midrange tone. Matching always produces the most power & most even frequency response.

If you are going to mismatch VALVE amps it is actually less bad, (for the amp or more specifically, the Output Transformer), to mismatch to a lower impedance, (16 ohm into 8 ohm cab), rather then higher. Unlike solid-state amps, valve amps are basically self limiting current wise (the valves!) into a lower impedance, though the valves will take more wear. Into a too high impedance the risk is different – potential very high flyback voltages can fry the OT. But most valve amps with strong output transformers will take a 1/2 to 2x mismatch without complaining.

Although not recommended, technically you can short circuit a valve amps output (0 ohms) without frying it – it is trying to put a signal into an open circuit (virtually infinite ohms) that is a real killer. Total opposite from solid-state amps of course – short circuits will kill the power transistors pronto, but they’ll sit happily all day with no speaker load applied.

Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) Develop less power as impedance increases. Do not short circuit (0 ohms) as this is sudden death for the output transistors.

Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low. This will wear the valves but the amp shouldn’t suffer. A short circuit (0 ohms) is normally survivable.

Impedance Matching

If mismatching high there is a risk to the OT, which increases with the severity of the mismatch, & with how hard you are pushing the valve output section. The ultimate ‘high’ mismatch is no speaker load ie infinite ohms. If trying to pass a signal into this then there is a severe risk to the OT from high flashback voltages which can arc through the insulation layers & burn out the tranny. Mismatching between 1/2 & 2x the impedance the amp ‘expects to see’ is normally problem free for most amps with healthy OTs. It is never guaranteed safe though, & being manufacturer specific Marshalls fail much more often when doing this then Fenders do. There is a lot of misunderstanding about impedance & mismatching issues, but I repeat that the advice I’m giving is correct for valve amps. (The exact opposite is true for solid state amps, but they work very differently, usually having no output transformers for starters!)

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes & leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.

Yes running 8 ohm amp –> 16 ohm cab is probably within normal safe limits (within 1/2 to 2x impedance range) for most valve amps. In fact you’ll often get away with playing Russian Roulette by running a 4 ohm amp to a 16 ohm cab, but the risk to transformers is definitely greater when going into a higher then intended impedance. You really are always better to have the speaker impedance lower if mismatching! If you value your amp that is. Going lower strains the valves more then normal, but they are disposable in a way that the output transformer isn’t. (Which is after all why valves are removable from their sockets.)

The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, & acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer & short the windings. I would not go above double the rated load on any tap. & NEVER open circuit the output of a tube amp – it can fry the transformer in a couple of ways.

It’s almost never low impedance that kills an OT, it’s too high an impedance. The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible – not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

Impedance Matching

There is magnetic leakage from primary to secondary & between both half- primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, & the spike is sitting on top of B+, so it’s got a head start for a flashover to ground. If the punchthrough was one time, it wouldn’t be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, & eventually erode the insulation to make a conductive path between layers. The sound goes south, & with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, & now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper & hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT’s if two ways – if it’s a break before make, the transients cause punch through; if it’s a make before break, the OT is intermittently shorted & the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

Too high impedances on the speaker outputs are much more dangerous for valve amps then too low impedances are. You can short circuit the + & – speaker output connections by connecting them together. (This gives very low impedance, close to zero ohms.) Valve amps can survive this. But if you don’t believe me then try this: power up your own valve amp, unplug your speaker, take it off standby & crank that baby into an open circuit very high impedance load. (Fresh air) You stand a very good chance of frying your amps OT!

Speaker load impedances and reflected loads to the output tubes are all “nominal”. An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the “power bandwidth”. If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don’t behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that “everything is critical”. In some circuits, this may be the case. Tubes don’t really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says “this is good”, “this is bad” and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

Boss RV-5 Digital Reverb

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Boss RV-5 Digital Reverb

The RV-5 is a direct replacement of the RV-3 so it might seem disappointing that it only has 6 modes compared to the RV-3’s 11. The RV-3 was however a Reverb/Delay that only had 4 pure reverb modes. With the RV-5, the delay function has been dropped and its 6 modes are all different kinds of reverb.

Features:

Boss RV-5 Digital Reverb
  • Stereo input/output for compatibility with other stereo pedals
  • 6 reverb modes (Spring, Plate, Hall, Room, Gate, Modulate)
  • The Modulate mode detunes the reverb sound for added spaciousness
  • New spring reverb emulation offers realistic spring reverb sounds
  • New gate reverb taken from high-end Roland studio gear

Specifications

  • Controls: E.Level, Tone, Time, 6-way Mode Switch
  • Connectors: INPUT A (mono), INPUT B, OUTPUT A (MONO), OUTPUT B, AC Adaptor (DC 9V)
  • Current Draw: 50mA
  • Weight: 440g / 1lb. (including battery)
  • Nominal Input Level: -20dBu
  • Input Impedance: 1MOhm
  • Nominal Output Level: -20dBu
  • Output Impedance: 1kOhm
  • Residual Noise Level: -93dBu (IHF-A, Typ.)
  • Recommended Load Impedance: 10kOhm or greater
  • Recommended AC Adaptor: PSA Series

Labels

  • Dark Gray – Made In Taiwan

The RV-5 was sold from July 2002 to present.

Boss PS-3 Digital Pitch Shifter/Delay

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Boss PS-3 Digital Pitch Shifter/Delay

The PS-3 contains both a pitch-shifter and a delay. There is seven pitch shift modes, and three delay modes as well as an expression mode. The pitch shift level is selectable in 26 steps – 24 chromatic steps over a +/- 1 octave range plus 2-octave up-down capability. The delay is up to a maximum of 2000 ms just like the DD-5.

Modes

Boss PS-3 Digital Pitch Shifter/Delay
  • Delay, 32ms – 125ms
  • Delay, 125ms – 500ms
  • Delay, 500ms – 2000ms
  • Detune, adds a slightly pitch-shifted sound to the input sound to create a detune effect
  • Fast Pitch Shift, very little delay in pitch shifted sound. Good for slightly shifted sounds.
  • Slow Pitch Shift, more delay in pitch shifted sound but very accurate. Good for greatly shifted sounds.
  • Inverse Pitch Shift, a mode where the pitch shifted sounds are reversed
  • Detune & Detune, adds two slightly pitch shifted sounds to the input signal to create a fat detuned effect.
  • Detune & Pitch Shift, detune output from Output A and Pitch Shifted sound from Output B
  • Pitch Shift & Pitch Shift, Pitch shifted sounds from both output. Pitch A controls Output A and Ptch B controls Output B
  • Expression, A connected expression pedal will set the pitch shift between Pitch A and Pitch B

Specifications

  • Controls: Balance, Feedback, Pitch, 11-way Mode switch
  • Connectors: Input, Expression Pedal, Output A, Output B, AC Adaptor
  • Current Draw: 70 mA (DC 9V)
  • Recommended AC Adaptor: PSA Series

Labels

  • Pink – Made In Taiwan
  • Dark Gray – Made In Taiwan

The PS-3 was sold from March 1994 to January 1999.

Boss OD-2r TURBO OverDrive

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Boss OD-2r TURBO OverDrive

The OD-2R has the same internal circuitry as the OD-2. Tests has shown that the R model has higher gain than the original but sounds very similar. The OD-2R’s only new feature is the remote connector. It accepts and optional a latching footswitch, like the FS-5L, and can be used to switch the pedal’s turbo mode on or off.

Boss OD-2r TURBO OverDrive

Specifications

  • Controls: Level, Tone, Drive, 2-way Turbo Switch
  • Connectors: Input, Output, Remote, AC Adaptor
  • Current Draw: 16 mA (DC 9V)
  • Weight: 440 g (1 lb.)
  • Nominal Input Level: -20dBm
  • Input Impedance: 1Mohm
  • Nominal Output Level: -20dBm
  • Output Impedance: 1kohm
  • Residual Noise Level: -110 dBm(lHF-A, Typ.)
  • Recommended AC Adaptor: PSA Series

The OD-2r was sold from November 1994 to July 1999.

Boss NS-2 Noise Suppressor

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Boss NS-2 Noise Suppressor

The NS-2 can be connected in two different ways. You can connect the NS-2 last in your chain of effects. The output of the previous pedal goes to the NS-2 input jack. Wire the Output jack of the NS-2 to your amp. The NS-2 can also be wired like an effects loop, which allows you to kill your entire signal chain just by stomping on the NS-2. Take the Send output of the NS-2 into your first pedal. Take the output of your last pedal and wire it to the Return jack of the NS-2.

Boss NS-2 Noise Suppressor

The NS-2 uses an intelligent circuit to differentiate between the sound of the guitar and noise. The NF-1 simply cut all sound when it became weaker than a threshold value. The NS-2 attempts to let the sound of the guitar through while removing the noise. The NS-2 production moved to Taiwan December 1989.

Specifications

  • Controls: Threshold, Decay, 2-way Mode Switch
  • Connectors: Input, Output, Return, Send, 9V DC In, 9V DC Out
  • Current Draw: 20 mA (DC 9V)
  • Weight: 400 g (15 oz.)
  • Recommended AC Adaptor: PSA Series

Labels

  • Silver – Made In Japan
  • Silver – Made In Taiwan

The NS-2 was sold from September 1987 to present.

Boss DSD-2 Digital Sampler/Delay

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Boss DSD-2 Digital Sampler/Delay

The DSD-2 is a combined Digital Delay and Sampler pedal. The Delay has two modes, S and L. In S mode it is variable between 50ms and 200ms. In L mode it is variable between 200ms and 800ms. The technology is similar to the DD-2 and they are in fact sharing the same internal chip. When used as a sampler, two modes are available: Play and Rec/Play. In the play mode, a recorded sample sound can be recalled whenever the pedal is depressed or when an external trigger is fed to the DSD-2. In the Rec/Play mode, a new sound can be recorded while a previously recorded sound is being replayed. Recording and replay is archived simultaneously while the pedal is depressed. The sampling time is limited to a maximum of 0.8 seconds.

Boss DSD-2 Digital Sampler/Delay

Specifications

  • Controls: E.Level, F.Back, Delay Time, 4-way Mode switch
  • Connectors: Input, Trig. In, Output, AC Adaptor
  • Current Draw: 55 mA (DC 9V)
  • Delay Time: 50 to 200ms in S Mode, 200 to 800ms in L mode
  • PCM System: 12-bit plus analog logarithm compression
  • Sampling mode:
  • Frequency Response: Sample/Delay 40Hz to 7kHz (+1/-3 dB), Direct 10Hz to 60kHz (+1/-3 dB)
  • Recommended AC Adaptor: PSA Series

Labels

  • Blue – Made In Japan

The DSD-2 was sold from April 1985 to August 1986.

Boss AW-2 Auto Wah

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Boss AW-2 Auto Wah

The AW-2 took over from the TW-1 in 1991. In addition to the regular Auto Wah effect it has a modulation effect that periodically shifts the sound. This make the variations numerous and it is known to be quite hard to adjust the AW-2’s sound.

The RATE knob controls the amount of modulation. Turn off modulation by turning the RATE knob to min. The DEPTH knob sets the range of the modulation effect. In the min position, modulation will be off and the wah intensity will be controlled by the picking strength. The MANUAL knob adjusts the frequency where the modulation Wah effect kicks in and the SENS control sets the sensitivity of the wah. High sensitivity means that a wah effect is produced with soft playing.

Boss AW-2 Auto Wah

There are no known changes between AW-2s made 1991 to AW-2s made at the end of its production span in 1999.

Specifications

  • Controls: Rate, Depth, Manual, Sens
  • Connectors: Input, Output, AC Adaptor
  • Current Draw: 15 mA (DC 9V)
  • Recommended AC Adaptor: PSA Series

Labels

  • Silver – Made In Taiwan

The AW-2 was sold from November 1991 to January 1999.

BOSS RPW-7 Power Supply

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BOSS RPW-7 Power Supply

RPW-7 Power Supply

Boss Micro Studio Series Vol.1, 1986:

Supplies 700mA to power a number of Micro Studio Series units
When integrating several Micro Studio Series units into a single system, connecting an AC adaptor to each can be a very bothersome task. A solution is provided by the RPW-7 Power Supply, capable of supplying up to 700mA of 9V DC power. 5 DC outputs are built-in to allow connecting 5 or more Micro Studio Series units through each unit’s adaptor input/output jacks. Another attractive feature is the RPW-7’s ability to supply power to Boss compact effect pedals which permit use with the PSA Series Adaptor. With the same design and size as the other units in the series which employ a PSA adaptor, the RPW-7 fits a rack perfectly.

BOSS RPW-7 Power Supply

Specifications

  • Rated output voltage: 9V DC
  • Output current: 700mA (maximum)
  • Power switch: 1
  • Power indicator: 1
  • DC out jacks: 5
  • Power: 117/220/240V AC (50/60Hz)
  • Power consumption: 22VA (maximum)
  • Dimensions: 218(W) x 42(H) x 140(D) mm (8 9/16″ x 1 5/8″ x 5 1/2″)
  • Weight: 1.2 kg (2lb. 10oz.)

BOSS RCE-10 Chorus Ensemble

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BOSS RCE-10 Chorus Ensemble

RCE-10 Chorus Ensemble

Boss Pocket Dictionary 4, 5 and 6

A 100% digital chorus with a digital modulation circuit and 2-phase chorus circuitry for spacious, impressive chorus effects A Pre-Delay Time control allows doubling the chorus effect and a built-in effect equalizer helps create a precisely tailored sound.

BOSS RCE-10 Chorus Ensemble

Specifications

  • input Level/Input Impedance: -20dBm/1MOhm), – 10dBm/47kOhm
  • Output Level/Output Impedance: -20dBm/2kOhm, – 10dBm/2kOhm
  • Output Load Impedance: Over 10kOhm
  • Sampling Frequency: 50kHz
  • Sampling System: 12 bit quantizing system plus analog logarithmic compression
  • Residual Noise: Below -100dBm(IHF-A. Level Switch -20dBm)
  • Controls: Pre delay Time(0.5—35ms). Modulation Rate(0.1-14Hz). Modulation Depth. Effect Equalizer. Effect Level
  • Switches: Power (On/Off). Effect (On/Off). Level (- 20dBm/ – 10dBm)
  • Indicators: Power, Effect
  • Jacks: Input (Standard, Pin), Output A (Standard. Pin), Output B (Standard, Pin). Effect Remote (On/Off), AC Adaptor (In/Out)
  • Power Source: 9V AC by PSA Series AC Adaptor
  • Current Draw: 80mA
  • Dimensions: 218(W) x 169(H) x 44(D)mm(8-9/16″ x 6-11/16″ x 1-3/4″)
  • Weight: 900g (2lb.)

FW-3 Foot Wah

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FW-3 Foot Wah

The FW-3 is the successor of the PW-1. The pedal features a peak control that determines the range of the wah as well as a jack for an external on/off switch.

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